Kevin wrote:

I'm installing a basement subfloor, and am having trouble figuring out what
to do with the existing toilet hookup.

The closet flange for the toilet is set directly into the concrete and once
I add the subfloor it's obviously too low (2 1/8" to be exact). I can't
seem to find an extension kit bigger than 7/16", and supposedly you can only
stack 4 of those (leaving me short still).

I can't be the first person to do this... any suggestions other than ripping
up the concrete and extending the pipe?

Agree with Nick.
You need to find out the pipe size.
There is a flange which has a rubber expansion ring
which will probably work if the pipe is 4".

Jim