Alas the PLO round drags the variance up so much. From the variance and winrate viewpoint, I'd suggest making the PLO big blind equal to 1/8 of the big bet, the NLHE one (and possibly a new NLO/8 round) equal to 1/4 of the big bet and the buy-in range of 10-25 big bets, so 40-100 NLHE bb and 80-200 PLO bb, which suits the postflop nature of PLO better. This would allow recreationals to survive past the PLO round.

But maybe Stars don't want to implement it because it's hard to make everyone post the same amount in blinds when they're so different in different games and people join and leave all the time. What do you think about this change?

As is, the total variance is about (1 + 1/4 [from NLHE] + <a tiny bit from FL rounds>) / 8 ~ 0.16 of the variance of the PLO round alone (if it was played all the time instead of 1/8 of the time), so the former's standard deviation (the sq. root of the variance) is 0.4 of the latter's. So $4/$8 8-g, with 1/2 PLO, is roughly equivalent to $0.4/$0.8 PLO, and requires at least twice as stricter BRM as the FL games comprising it. An aggro / normal / tight BR size for $4/$8 8-g is therefore about 400 / 700 / 1200 big bets imo ($3200 / $5600 / $9600), maybe less if most 8-g players suck at PLO big time.

A related question I've had for long is how ethical it is to sit in for the PLO (maybe NLHE too) round only and then quit for another 36-42 hands while grinding usual PLO on the side

PLO seems the biggest part of the mix winrate-wise as well... or is 8-g infested by PLO regs already?