You’re right in saying a proper setup will mitigate the O2 issues, the issue is it’s likely OP isn’t providing that environment for the PBT in question, as far as tank sizes go, no people don’t overestimated or over blow tank sizing for tangs, those experienced reefers who have a tang in a smaller than recommended tank know what they’re doing, know how to control the aggression issues and have exit strategies, newbies just don’t abs more often than not end up killing the tangs or the tank mates.. this is especially true with acanthurus, Nasos have no business in anything less than 8 foot, they can quite comfortably get up to 1.5 foot in length and are open grazers in the wild.
I’ve also kept angels in my reefs and they are different to tangs in how they behave, a pomacanthus will have its cave or outcrop and rarely stray from it, acanthurus, Nasos and hippos all tend to swim across sections of reef grazing, heck, they’re built to be more hydrodynamic than beefier angels and other more sedentary fish, and this should explain it all as to why they really need A larger tank.
I had a 260 with a desjardini and purple before I downsized to a WB reef100.3, atm I’ve got a tiny ctenochaetus binotatus in QT who will be cleaning the tank with some other open swimmers like dispars so my skimmer actually get some loading haha! This tank is way too small for an adult and I have an exit strategy if I haven’t gone back up to a 200 gal by then, hopefully bringing my desjardini back into the bigger setup! A 120 will work for PBTs for about 6 months, and then if things work out the rule is that one by one you other fish will be beaten to death, this rule applies with most all other acanthurus tangs, the exception is they play nice haha!
edit: that being said, ‘the best we can do is the best we can do’ as hobbyists, we should always strive to either keep tangs in big tanks or be disciplined with what we chuck into a reef with a tang, and PS I’m not a huge fan of tang police either haha!