A little update on the whole project.
I've since blown the welder up two more times.

Each time getting closer to proper operation.
With the new diodes i have (IDW100E60) and new MOSFETS (IRF460) the welder does power up and survives a trigger pull just fine.
No load with the trigger pulled (operational) the inverter draws 50mA from the mains, which i think is not too bad.
But the MOSFETs popped as soon as the welding wire touched the metal scrap i tried to weld.
I then doubled up on the MOSFETs, previously there were one per side, now there are two per side (a total of four) and also removed two turns off the transformer (10 primary, 5.5 secondary).
Did some tests, everything seemed ok-ish, i then separated a few strands of my negative welding wire and shorted them out to the positive to see how well it melts wire.
It lasted about 30 seconds before blowing up yet again, although a lot more spectacularly than before.
So for once i decided to do some actual research and calculations (groundbreaking stuff, i know).
Couldn't find much info on FBT cores, but B
max on them appears to be in the range of 2000 to 3400 gauss.
I took 1500 as my B
max to be extra safe and calcuated a primary of 10.7 turns (rounded off to 10, because generous B
max estimate).
From that i calculated my secondary, which turned out to be around 3 turns.
I'll rewind my transformer in the same way i did before with the primary between two secondaries, but i'll split the secondary wires to make them bifilar.
So my transformer will be wound with the primary between two bifilar secondaries.