Should this metal part be screwed or it can safely be welded?

Do a hole-spot-weld and then smooth it with a wheel sander.

To keep the outside clean you can use Spot Weld Studs and screw it from the tube section with the clearance for the nut/wrench.

You may consider using 3M automotive grade double-sided tape.

In my country it is very common to join aluminum or fiber sheets against steel pipe structures, to create bus bodies.

Greetings

"PS: I see no way to not notice the heat affected zone of the solder..."
So even on a 2 x 2mm metal sheet, the outer face of the part will have the spot welding visible?

Regarding the welding of the tube with belt, I will have an intermediary corner so it can be weld from the top, no need to go through the tube.
Thanks

I have a client who manufactures aluminum structural profiles for glass fronts in all types of buildings. Some glasses are framed but others (they call them "glass walls") are directly glued. I pass you a part of the catalog that shows the assembly sequence and the adhesive materials used. Keep in mind that, in the south of Argentina, we have one of the most powerful wind zones in the world, and for it we make the calculations with wind speeds of 245 km/hr (68 m/s) and they generate enormous pressures, of around 288 Kg/m2 both in pressure and in suction.

I'm not saying you have to "have confidence" in industrial adhesives, just "know about them". Because if you think that in all fastening applications the best thing is a screw, or a rivet, or a weld... you are leaving aside other resources that even have better behavior than those when faced with changes in temperature, exposure to humid environments, VIBRATIONS AND FATIGUE!!! (Which are two of the phenomena that make us mechanics lose sleep).

The mechanics are very broad.
Kind regards!