Gonagul wrote: ↑Fri Jan 15, 2021 1:33 am
I want the style

of a single-weapon rapier rogue.
Thought it might be neat to have a combat character that was flexible enough tactics-wise to fight in whatever way I'm in the mood for that night. So was thinking about maybe some sort of 24rog/6rng archer path build. Can kite, can trap, can grenade, can hit and run-- and can also maybe stand toe-to-toe in parry mode?
It's actually a very nice idea, but it is a bit tight with feats. You will probably have to choose whether to privilege archery or melee with your feat picks. Also, I would go for 25 rogue / 5 ranger, since level 25 rogue has an extra sneak attack die. Your mileage may vary though, ranger 6 gives you animal language and that is cool too for rp.
Am I going to have a terrible time giving up TWF?
You are going to be all right, really, even if probably rogue with shield, fencing buckler specifically is more optimal, if anything because it gives you an extra gear slot and frees up 30 skill points.
Does rog/ranger get required BAB for full parry bonuses?
Yes, you need to take 4 ranger levels pre-epic, so that you will get 21 base attack bonus at level 29, which will unlock the +6 armor class from parry.
Can GSF: Transmutation be squeezed in?
Your feats will be tight, but if you can fit it in, why not.
Would max parry ultimately be a crappy use of skill points? I've admittedly never used parry, and don't know what skills it might ultimately replace.
If you care only for the armor class bonus a shield may be more optimal, but let's be honest, a one-hander fighter looks cooler.

Ultimately if you do not need any extra skills it will be all right.
Parry by itself can have niche uses but it is ultimately not very worth it, since you can parry maximum 3 attacks per round, and especially against groups the parried attack is random, so you cannot riposte a single enemy at once. In some pvp contexts it is nice, but it requires heavy gear and feat investment, and people will just stop attacking you or will pop true strike. Incidentally adamantine bracer of avoidance, a popular gear piece among dexers, has a +6 to parry.
The bottom line is: if you like the style of it, do it.