You can definitely use a SNOO past eight weeks!
The recommendations to stop swaddling are NOT because swaddling prevents a baby from developing motor skills. You only swaddle when the baby is sleeping - that's not when they're developing those skills. The recommendation to stop is because as those motor skills develop, you risk the baby rolling over and getting stuck on his or her tummy, no arms, and being in an unsafe sleep position without the ability to self correct. (
https://www.webmd.com/baby/when-do-you-stop-swaddling-baby#1)
In the SNOO, this is not an issue, as the special SNOO swaddle is strapped down to the bassinet - the baby cannot roll when in the SNOO. The SNOO people recommend weaning off at 5-6 months (
https://www.happiestbaby.com/blogs/snoo/transition-baby-to-crib-snoo-weaning-feature)
Personally, we did a one month rental to get through a terrible sleep regression before my baby was big enough for sleep training. We did a sudden wean right at four months to do CIO and move baby into his own room and crib. My kid was a late roller, but I don't think that was at all related to his month in the SNOO, but rather because he had reflux which made tummy time really hard to manage (had to hold him upright after feeds, etc) so he didn't have as much opportunity for "exercise" during his wake time.