I use a Mac for work which means in most (split/ergo/tenkeyless) keyboards (depending on the settings applicable) either Ctrl or Alt/Opt is missing from the right side, because e.g. an Fn key or a squeezed-in left arrow is taking their place. I need all 3 modifiers constantly (Ctrl+C in Terminal, enter special characters, use app shortcuts, jump between windows/tabs...)
I have an ErgoDash, which has several thumb keys on each side — the size of your hands would determine how many are practical for frequent use.
My squeezed-in left arrow key serves as Alt when I hold it down, and Enter is Ctrl if I hold that down.
More importantly, I also really haven't yet found a "mechanical" keyswitch (== with a Cherry MX mount) that wouldn't be worse for my RSI than traditional rubber domes (lately I've especially liked the low travel ones, currently Logitech Ergo K860). Sure, it may also be more about the design (or lack of design) in many of these "mechanical" keyboards I've tried that is bad for my specific kind of RSI – it's hard to pin down.
Unfortunately, if you want rubber domes, the options are really scarse. I wish the religious fandom around Cherry MX style switches would decrease in the upcoming years, to give more room for new kinds of keyboard innovations.
There are some low-profile MX mount mechanical switches [1], but I have no experience. I did make the gallery [2], but it only has mechanical keyboards – I think other sites cover rubber-dome keyboards adequately.