Steel/Aluminium design/build, sorta smallish, cuts ali hopefully

[JAZZCNC]

I was already designing some jigs in my head to keep weld distortion under control. Now I can probably ditch those.

Jigs to stop distortion are not practical really.? Heat cause's distortion and stress so to restrain these forces would need massively strong Jigs which for a one time machine just isn't worth the trouble. If you stitch weld in short runs and spread welding around so not to create heat build, Take your time and don't try to weld frame in one go let it rest and come back to it then you won't have much trouble.

Tips to help with stress relieving are Stitch weld and balance the welds IE: If you weld for half inch on outside face of steel box then immediately weld half inch on inside face opposite to keep balanced stress's. Don't weld all of one side up then turn over and do other, Also Don't weld in one spot for long but Move around and try to weld with minimum heat.

But with the Y. I basically just copied some designs I found here and there and I don't really have any more ideas. :(

I'd just move the rails on Gantry closer towards front and just have narrowest top and bottom brackets possible you won't need any thing more than that and they will be more than strong enough. Edit: When I say narrow I mean looking from side not narrow bearing spacing.!!

I noticed your Z axis plates look on the thin side what thickness did you have in mind.? For the front and rear Z axis plates I wouldn't go less than 15mm and 20mm is best balance of strength and cost.