There are also many that use the right hand for 6.
There's no real solution to this. There's always people who will complain about the 6 on the right side and others about the left.
If there's a solution could be to have one on each side (or one big enough to cover both sides) or being able to change the 6 side"on the go" somehow.
You might have difficulties sourcing a 2.5u key for the right Space bar.
This is surprisingly hard to find. You will find 2u, 2.25u, 2.75u and 3u, but 2.5u is a little impossible.
EDIT:
Why the inverted T cluster on the left side?
I initially thought it was for "nav" cluster (pg up, pg dn, home, end) but you put the nav on the middle.
BTW, I don't think the Nav cluster is so important to be put in the middle.
Good keys in the middle could be:
Shift, Ctrl, Esc, Tab, Enter.
I also don't get why the Alt keys are so big.
You could make Ctrl bigger instead, or leave those 3 keys with the standard size 1.25u.
If the the 3 of them could be 1.5u it would be nice, but just Alt is nonsense to me.