I believe that this mixing things will not greatly benefit you.

The real things that should be considered are: precision, strength/bend twist/, weight , ball screw length, price.

What i mean is the aluminum will raise the price, lead to lower precision and a lot of hassle in combination with the steel profiles, and possibly raise even the weight. What you will gain is beauty.

I dont know what precision you aim for, but any imprecision in the aluminum will transfer to loss of precision and need to adjust.

So from my limited understanding, either use steel and epoxy or use precisely machined aluminum for all the gantry, like Dean makes his ones.

Aluminum will not magically solve rails alignment as it seems you believe, unless you buy specially intended /$$$/ and precisely machined /$$$ or ability to do it/ .

Combining it with steel will lead to shimming, what you on the first place seem to try to avoid.

If you decide on the steel only machine, i can even help you and arrange all parts laser cut in Bulgaria for cheap, so you can drive 200km and get them, assuming you live in Greece and not very far away. I mean all parts on the machine, like i did from 10mm steel sheet. That can give you immense freedom with the design and extreme ease of assembly. Not to speak of avoiding hours and hours drilling precise holes.

Just for some ideas, you could take a look at my second build. i believe the design of my big machine gantry is even better, avoiding the laser cut parts and aluminum parts as much as possible. And frankly i have not seen a DIY gantry with less overhang. See post 34. You can ignore the extreme Z as there will be 12kg spindle or pneumatic hammer, but take a look how the motor fits, how the gantry sides are avoided to be expensive and heavy. How easy will be to epoxy level the gantry rails as they are in one plane but at the same time there is no overhang at all.