awwww, Marvel Straxus got into the Japanese Cartoon Continuity, that's cute...
tbh even tho Transformers doesn't have a main universe, I always tought that 3 could be that spot the IDW Hasbro Universe (But it ended so rip.), The Alligned Continuity (But it isn't relevant as before), and ofc the Japanese G1 Cartoon, and tbh with you guys, thanks to all the multiversal crossovers that are in this continuity (yeah Straxus is only one of them, even Slipstream from Animated got here) I think they should make JG1 as the main canon.
EDIT: Since Megatron time travels to the universe that existed before the actual one, in an hypotetical Simplified order we can place the stories after the war on Earth.
And little note, since Megatron appears in the 1st universe, he's technically the 1st Cybertronian, even tho he is an assembled one and not a pure one.