Ahhhh my favorite topic
You seem to have most of bases covered, I agree 2 fingers on the g-spot on the upper wall in addition to a tongue on the clit works wonders

Forcing 2 fingers open into a V shape and sliding in and out creates wonderful amount of friction against the lips of the pussy and walls, also try going vertical with the V too to hit the top and bottom pussy walls and the corner of lips below the clit. I think it gives her a very nice full sensation
Teasing by alternating oral and then stopping and inserting penis for a few minutes and then back to oral. Drives my girl wild after a few cycles of that because she never knows when I'll finish her off.
I also enjoy keeping her cumming as long as she can take it and then pausing for a few seconds so she thinks it's over and then bam back in with tongue for 2 seconds and she quickly pops all over again
Another orgasm extension techique is after she's cum enough place your hand plam down and cover her pussy and add a little pressure and let her ride your hand for a minute. Kind of keeps it going but at a lower cool down pace.
Most important thing is be creative