I have always had difficulty finding good labs that could do a reasonable job with my negatives, which is one of the reasons I prefer to shoot slide film.
I would suggest you make a list of your lab info requirements ( like you have in your OP) and then you can email the different labs with this info and see who responds and does their response meet with your approval/requirements.
I processed C41 films commercially for over 35 years (I am retired now) and the difference between labs is down to how tight and tidy they run their operation. C41 is C41, but how your film is handled is a different matter.
Any lab worth calling themselves a lab (mini or otherwise) should take pride in their work and produce good and consistent results. In the bricks & mortar days, a simple visit to the lab would tell if they were good, by simply seeing if the place was neat and tidy and the processing machines haven't chemical splash and dribble marks all over them.
Also, someone eating a sandwich/drinking coffee while scanning is not a good sign.
And then you have the scanning, different people prefer a Frontier, a Noritsu, or a Pakon. Having used Frontiers and Pakons, I personally think either is good (but then I am biased). It's a bit like the question of which camera is better. To be really truthful I would be hard pushed to tell the difference between a scan from any of the three.
Good scanning is down to the operator and not the machine. Good scans are not done on full auto and are reviewed on the monitor screen and an experienced eye will make any corrections manually. As the name suggests, an experienced eye is only gained by experience and also knowing what their customers expect and want.
I would think a simple rule of thumb for gauging a lab is by how busy they are. If they are not busy, then there must be a reason.
