Any mechanical keyboard supporting QMK is fully reprogrammable, so you can either choose one with sufficient keys, or enable one of various options to double-up particular keys.
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.
Among the QMK keyboards, it's still very tricky to find a tenkeyless/ergo keyboard with mostly a normal layout (with enough keys between the spacebar and the arrow keys to host all 3 Mac modifier keys).
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.
Ah, sorry, I was thinking far too much of the mechanical keyboard nerds when you wrote "ergo". My ErgoDash is a few steps weirder than the Logitech Ergo K860. I find those low-travel keyboards extremely uncomfortable for typing; being unable to replace a much older full-travel ergonomic Logitech keyboard is what drove me to a mechanical alternative. I did find it uncomfortable at first, until I realized I did not need to press the keys anywhere near as hard as I was used to.
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.
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.