Thanks for the shout-out, I have a good bit of experience welding aluminum, but I have to admit the metallurgical side is not something I’ve studied enough to give sound advice.
Most of the aluminum I’ve welded over the years is not destined for heat treating, or I was simply the welder. I.e, I didn’t design the thing.
I follow the mindset of ‘lot of extra work, for not a lot of gain’ in the context of aluminum bikes. But of course that’s just my opinion.
But it sounds like @Dan_Chambers knows his stuff. I don’t have any experience with the bicycle specific tubes or alloys.