There are probably many ways of doing it at software level, I would suggest either of these two cross-platform tools:

[1] https://github.com/kmonad/kmonad

[2] https://github.com/jtroo/kanata

With the extra keys, you can have 4 modifiers on each side of the space bar: Control, Win (Super), Alt and Cmd - total 8 modifiers, ideal for your thumbs!

This still leaves you with the Yen and the Ro key to map for other things!