As above – any recommendations? Something like Waxoyl?

I always do using Fluid Film AS-R. And don’t forget to drill a hole in the bottom bracket shell.

It’s completely unnecessary. But if it makes you feel better, then knock yourself out

Try weighing the frame before and after. Waxoil adds quite a lot of bulk and isn’t needed at all IME.

Waxoyl is overkill.

if you wnat your frame to last forever sue soemthing, but not waxoyl

And dont drill a hole in the BB, as in my experience it kills bearings much quicker.

One problem with Waxoyl, or similar, is if you get your frame sprayed in the future, then, when they stove it, it melts, runs out of the drain holes and ruins the paint job (saw it happen to an Orange many years back.).

no need to waxoyl or anything. Steel used for frames is not really susceptible to massive corrosion due to the alloy used and treatment of it.
You have to really abuse a frame for it to corrode in this way.