It took me a while because of the ambidextrous bolt releases (I forgot how to put it back together and needed to go hunting on Youtube), but it's functioning again.
I will admit that a sense of impatience did overcome me toward the end, thus the shimming is probably "good enough" rather than "good". A few test shots show an FPS of around 285 on 0.25g with no hop. This did fall down to 275 on a second test - not sure why, but the fake bolt got a bit stuck around this point and wouldn't seat itself all the way home, so possibly that indicates something.
Almost certainly the air seal could be better - I thought chucking money at it all (see below) would result in something consistent, but I should probably take it apart again soon and see if there is any wear on any parts where there shouldn't be. I've done no AOE correction - it didn't look like it needed it (and I've been down a bit of a rabbit hole with regards to if it is really necessary...)
For my own reference, the parts swapped are:
Falcon ETU > Perun Hybrid V2
Stock spring of unknown strength > XT M105 Spring
Stock bushings and bearings > Zero J-Caged 8mm Bearings (potential clearance issue with the selector plate if I were to shim the gears perfectly? but no issues so far).
Stock 229mm brass barrel of unknown diameter > XT 229mm stainless steel 6.02mm barrel
Stock rotary hop unit > G&G rotary hop unit
Stock bucking > Maple Leaf Macron Green
Stock nub > Omega nub
Stock piston > XT 14 Tooth Piston CNC
Stock piston head > SHS Piston Head (6 Vents Aluminium)
Stock cylinder head > XT Cylinder Head Aluminium V2/V3 (Wide Pad)
More shims added (stock DE M906C has x4 0.5mm, x4 0.2mm, and x2 0.1mm unless I've miscounted)
I attempted to add an SHS Safety Latch (V2) but the selector switches really didn't like it, so I gave up on it (and it's not necessary thanks to the Perun).
Hopefully soon I'll get somewhere where I can test this out at range. As for upgrades, the next thing has to be a motor - the stock DE/Chaoli(?) motor is lethargic at best (and I could feel it getting every so slightly warmer than the ambient temperature after just a few shots). Dunno if I need to go brushless tbh - I'm more woodland focused than CQB, so I don't need the snappiest of snappy trigger responses.
Anyone have any thoughts on how to monitor how the new parts are all interacting with each other? I'm probably gonna have to take it all apart again relatively soon right?
(...oh crap I probably haven't put enough lubrication on the gears lol)