I like putting backwards delete on a thumb key, given fully programmable firmware / a key layout that can fit it (Ergodox, etc.).
The Colemak people put it on caps lock, which also seems like a good choice.
If you don’t use brackets ever, putting backwards delete on [ seems like a reasonable choice, though.
Have you considered down-sizing to a 40–60% keyboard? For instance, I think this would work pretty well:
