I’m wondering if there’s a way [to] combine their computational power.

Only if your problem can be be split up reasonably, otherwise you will spend more time waiting for data to move.
Where it can work: video encoding, CI pipelines, data analysis
Where it won’t work: interactive stuff, most single file operations

I want to get into server hosting […]

Then you don’t need another reason to do it.

[If] I can connect them all to one display [or] make them all accessible in one place.

You can either get a hardware switch or chose a primary computer and connect to the others. For that you can use remote desktop software or be a try hard and use ssh.

Because the rocket equation is brutal.
A rocket is almost entirely fuel some structure and a minuscule amount of payload. If you want to save fuel for for relanding you need to take less payload or accept a lower orbit.

If we look at Falcon 9, the payload to geostationary transfer orbit^* goes from 8,300 kg when expended, to 5,500 kg when landing on a barge or 3,500 kg when landing on the launch site.

* meaning the payload needs an additional stage to enter geostationary orbit

Just put it in a formal grammar:

GNU -> GNU's Not Unix's
GNU -> gnu
GIMP -> GNU image manipulation program