VC17 isn't the do all end all, but it IS a pretty simple bottom paint that does a nice job. None of these paints are worth a darn if you don't prep the bottom correctly.
The truly OCD racers use epoxy bottom. My boat is a slip filler most of the summer... some kind of active bottom paint is required for me... although our lake is a very low growth lake. VC17 is preferred, because clean-up at the end of the season can be done with a sponge. We have no pressure washers, or lift, or crane... so it's 5 minutes at the ramp with a sponge, and work is done.
Sadly if you had ablative on the bottom before, and it's failing, to get the bottom proper smooth again, you'll need to scrape/sand/barrier coat, or blast/sand/barrier...
Either way it's hard work (or expensive) and quite toxic. I've done it, but I won't do it again if I can avoid it (I'd be dragging the boat to a soda blaster first before I tried doing it again myself).
I like and still use VC17m.. not because it's fast, but because it works, and its easy to apply... and you can do it year over year, and it takes like 15-30 minutes to apply another coat.... and you can put on layer after layer for quite some years without it turning into a bumpy landscape. I know nothing of SR21, but imagine its very much like VC17m... But again, either would be sufficient in freshwater if you have a decent smooth barrier coated bottom, that is where ALL the work is.
Bottom stripped, sanded, and fairing beginning
VC17m 4 coats, 2 red, 2 copper