Thank you for the reply.
This explains my post from last summer: https://forum.fanatic.com/d/26889-blast-footstraps
And I just saw this: https://forum.fanatic.com/d/27111-blast-undersized-hole-update
The footstrap screws that came with my 2020 Blast definitely are M7, not M6. This is probably why I've found it to be very hard (but not impossible) to screw them in. I think I will try the m6 x 28mm, although maybe they'll be a bit loose because of me using M7.
For what it's worth, K70 PT is the thread specification for M7 plastic thread forming screw. The M6 x 28mm would be a K60 PT and the M7 would be a K70 PT.