In 2017 to use with my Canon A-1, I picked up a Canon FD 100-300 f/5.6 telephoto lens and put it on a Canon 2x-B Extender which doubles the focal length (but kills off two stops worth of exposure). Thus the zoom was effectively 200 to 600mm. For my EOS 40D I snagged a Canon EF 70-300 f/4-5.6 zoom. With the 1.6 crop factor that reaches to a 480mm equivalent in 35mm full frame terms.
In 2017 I noted a few "things to remember" - - -
a) At those focal lengths, the sun moves across the frame surprisingly quickly. I suppose if one is wealthy a clock drive might be cool. I guessed in the future a 3-axis geared head could likely be set up to to have one axis lined up to track the sun. Just received such a head, we shall see.
b) Also used was a right angle viewfinder attachment -- turns out the FD flavor works on the 40D so I bought two for less than an EOS flavor. Did find that eye relief and positioning was tedious. Another viewfinder problem is that with a 16-stop (or whatever it is) ND filter it's nearly impossible to really observe a frame reference to allow centering the sun accurately.
c) Recommendations were to tape the focus ring so as not to mess up focus in the dark. Sounds good, but both of the zooms I used were the "one touch" sort that I couldn't actually tape -- because if the sun moved out of frame, I needed to widen the view to even find it.
So this year (subject to lake effect snow, a delayed flight from overseas the day before, etc., etc.) I am attempting to address the above issues without totally breaking the bank:
1) Going to drop to just one camera, my EOS M-5. I can place the EF70-300 on it with an adapter I already own. The M-5 has an articulating rear LCD screen, no need for right angle widgets.
2) A 3-axis geared head, and a hefty 3-section tripod that can handle 12 pounds or more has been on the wish list for other reasons. FedEx left it here yesterday.
3) Perhaps better than a cable release, I have put the Canon Connect app on my iPad and can shoot in wireless tethered mode (with a live view on the iPad screen). Not having to futz with anything but one occasional axis adjustment at the tripod should reduce "handling" according to The Plan(tm).
4) Only needing one, a foolish amount has been spent for a Hoya solar filter which may be better optical quality than the cardboard and metallized Mylar units used in 2017.
5) In Illinois our base was a lawn outside of a refreshment/restaurant/gift emporium which was nice because we arrived early and spent quite a few hours there. This time we will (hopefully) be day tripping to some park or wildlife refuge for a shorter stay
6) The usual array of multiple fully charged batteries and backup battery supplies will be carted along.
So-o-o, we shall see. I will likely put gear for one of the previous setups in the car as backup.
[Ooops, cheese, didn't mean to write a book, but perhaps it will help others figure out their plans ...]