Kazzo another option you may have is, I know you said you can't put anchorage points in (to the floor I'm guessing), but you can get a CRAB installed.

DH had one installed into his car, never used it as I put my bigger 2 in the third row with untethered boosters, DD3 and DS in the middle row where points already were.

With the CRAB, we can now have 6 tethered seats