put enough air to stop them bottoming, with -ve air set to 0psi, then once you've sorted that, pump up the -ve air.

The more -ve you use, the more sag you'll have. I usually pump mine 5-10 psi above the +ve chamber as you lsoe a bit when you take the valve off.

there shouldn't really be a need to fiddle with damping oil etc..

yep

As you state, your options are:
* less neg pressure, effectively preloading the fork and raising the ride height

*fitting a poploc adjust/using non poploc damper and using the LSC/floodgate to control the forks movement

Why don't you want to use external compression adjustment? If you've got the "right" amount of sag and you're blowing through the travel then I think most people would advise some compression damping.

If you want to make the adjustment on the spring side, then you could try adding oil to the +ve air chamber. A smaller air space should ramp up faster for a given amount of sag.

Lastly, are you setting +ve pressure by reference to measured sag, or are you following RS setup guidelines? New seals and general new fork stickiness might, depending on how you position yourself on the bike to measure sag, give you a misleadingly small amount of sag or a low pressure.