OK, some thoughts from my end on getting started with RA4 for anyone who's planning to:
About paper: get some fresh/new paper. Don't waste your time on old paper or paper from obscure sources. You'll spend months troubleshooting your process only to find out your paper is bunk. By all means try some old paper once you know what you're doing.
About chemistry: use whatever is available. In the US, Kodak is apparently easier to get, here in Europe Fuji seems easier. Minilab chemistry is the cheapest option on a per-liter/gallon basis, but you may want to try with smaller volumes first. Read the manufacturer's datasheet for your specific product. Consider replenished developer and blix instead of one shot. It's economical and dependable.
Get a pH meter. Skip the cheap $10 yellow or blue ones (the bar-formed ones with the vest pocket clips), as I find they're hit and miss in terms of reliability. Spend twice that (still not much) for a halfway decent one. I got myself one of these and it's good enough (and quite a joy to use):
https://www.aliexpress.com/item/32962476637.html
Periodically monitor particularly developer pH, and also verify it when mixing fresh dev and/or blix. I find that developer pH is by far the most critical parameter in the RA4 process, even though deviations of +/-0.05 often don't cause big problems (unless you're hyper-critical), but it's amazing how violently a replenished developer can sometimes shift. Get it back in line and it works like new, even though it looks like last week's coffee with a hand of dirt thrown in for good measure (seriously, a long-term replenished RA4 developer can look like something that does not belong in a photographic darkroom and resembles the stuff that seeps out of a farmyard dung heap more than a developer).
Stop bath: Use one. Store-bought acetic acid is fine. I use citric acid myself because it's odorless. Supposedly citric acid should not be part of a color process, but I have never learned a specific reason why, and neither have I experienced any ill effects from it. Stop bath strength is 1-2% or so. Not critical. As soon as prints don't start feeling tacky after a few seconds in the stop, discard it and mix new.
Development: I'd suggest using trays and room temperature, because the step-in costs are virtually zero and the result is right on par with roller- or drum-processed prints. You only have to learn to navigate in your darkroom without any light, but you'll get used to it surprisingly quickly. Also see tip below.
Darkroom navigation: some people like a safelight for color, but I'm not one of them. For navigating my darkroom (and preventing myself from bumping my head into the enlarger head - raise your hand if you recognize this), I went out and got me some of those neat little fluorescent stars that go onto kids' bedroom walls and ceilings. You know, the kind that gives the dull green light in the dark. I cut some of those into smaller bits and pasted them into a few strategic locations using double sided tape. Don't let them shine their light (even though it's quite dim) directly onto the paper, so choose the spots wisely. But I find this really helps in getting around in the dark. It doesn't have to shine light onto anything; it's just a tiny little beacon to navigate by.
About exposure/filtering: get the exposure time in the ballpark before narrowing down the color. There is usually some color shift between large variations of exposure. It's negligible between small increments/decrements in exposure. Color filtering: you may be tempted to try inching your way towards an appropriate color balance by adding/removing 2, 3, 4 or 5 units of Y or M. Don't do this at the start. Begin with adjustments of let's say 10 or 15 units on a single filter and make strips that are blatantly too green, magenta, yellow and blue, only to narrow down once you've done that. That'll help you to develop a feeling for what kind of bandwidth you're playing with. After a while it becomes second nature to guesstimate how much filter change a print needs.
Test strips: don't make them too small. Make sure they cover the full contrast range and preferably the most challenging colors in a negative all at once. As soon as you get close in terms of exposure and color balance, expose a full print - don't try to go all the way with test strips before dedicating a full sheet. Consider this: processing a test strip costs just as much time as a full sheet. But RA4 paper is relatively cheap. What kind of financial value do you put on your time? Does saving maybe $5 in paper in a week-night printing session weigh up to the time lost by having to do another round or two of filter optimization on each image?
Evaluating test strips: tape a neutral grey scale to your darkroom wall and hold the strip next to it, especially if you're trying to print something that is close to neutral in color (clouds, overcast skies, concrete, etc.) If you don't use this kind of benchmark, you'll be surprised how quickly your brain adjusts to the most horrible color casts telling your mind you're seeing 'neutral'. The eye/brain has some seriously effective auto-white balance functionality, but it gets in the way if you're printing color.
Dry strips/prints entirely before judging them. Some papers show only a marginal dry-down (e.g. Crystal Archive), others undergo a huge transformation as they dry (e.g. Endura: the blacks seem a hazy blue when wet, but become deep and perfectly black when dry, and the reds seem dull in a wet print, but gain their unique saturation and depth only when dry). A hairdryer is great for this (I usually use it on finished prints as well since I'm just impatient like that). For test strips, don't worry if you don't blix for the time indicated by the blix supplier. After about 20-30 seconds, most (or even all) of the blixing has already taken place and a test strip/print can be washed quickly and evaluated.
Darkroom lighting: I installed cool-white led strips in my darkroom a few months ago and it's a Godsend. Even though their CRI is probably not optimal, they still make it really easy to judge test strips. In any case: don't judge test strips under normal incandescent bulbs let alone under CFL's, and I'd stay away from warm-white leds as well. Get a light source that is fairly close to daylight. Make sure it puts out quite a lot of light too; you can't judge shadows under a weak light.
I put my lights on a remote-controlled switch, I keep the remote in my pocket so I can quickly flip the lights on/off as desired. Lazy, but I wouldn't want to go without now I've got this. You'd be surprised how often you find yourself walking to that light switch during a typical printing session.It becomes old pretty fast.
Final word:
I'd recommend just making some good straight prints without any fuss, tricks, etc. Make sure your negatives are good; preferably start out with negatives developed by a reputable lab, properly exposed, and on fresh film from one of the major manufacturers (i.e. Kodak/Fuji). Don't assume that 'well these negatives scanned really well so they should print well too.' Leave any digital experiences at the door. You'll fool yourself if you believe that scanning bears any resemblance or is any proxy to darkroom printing.
If your negatives are somewhat decent and your printing process is within reasonable limits or reliability, you'll find that 95% (or more) of your negatives print just fine without the need for advanced trickery. Could yet better prints be made? Absolutely. Techniques such as masking open up a whole new world of possibilities - but the magic of C41+RA4 for me is that it's relatively easy to get a really nice result without all that much effort...once you get the basics down.
