You don't like to hear it and i would not say it again, but one last time for the benefit of all people who will read that in the future:

-50x50 is not right for a machine that will mill aluminum, 80x80 or 100x100 3mm is the right profile to form the gantry and the sides or the whole frame

-saving on box section size=5 times more cutting and welding diagonal braces, where if 80x80 or 100x100 there is no need for diagonals and simpler construction

-there is no such thing as 0.1- 0.3mm accuracy, especially cutting aluminum. If the machine is made right it will be repeatable 0.05mm and accuracy will be ~0.01- 0.02mm
0.3mm accuracy is for MDF machines or crappy cheap machines, in other words trash that will not cut aluminum at all.

-Modeling wil get you to nowhere, only wasting your time. Check what people build on forum, choose a build, see the results and imitate it. Then you will be successful. Make a build log, ask questions and listen to them answers..

-use common sense, if you lack it, there is no software to help that. Go to a local welder and find a 2m piece of that 50x50 profile, 80x80, 100x100, touch it , try to bend it then for 1 min everything will become clear to you. 3m 100x100 is flexible and 50x50 is like a macaroni

-every crappy machine can cut aluminum, scratching or slow, but the better the machine the most perfect the finish will be.

-if on a build of 60x90x20 for example machine you think that 10cm more or less is a great thing, better not start it. 10cm of rail and steel more or less in abuild are no more than 100-150e, and if you dont have 3k for parts and 500-800 for tooling at least, you are in the wrong place.