I know people have done copper pipe tubing and it can be a pain to bend the pipes. I know you can buy softer copper that is more malleable but even then it seems like more trouble then its worth.

My question is could you solder on fittings for bends and such, or would the solder used cause issues in the loop? I've done plenty of copper plumbing and soldering fittings is not hard at all for me at this point so it seems to be a much easier method then trying to bend the pipe.

I'm just curious as I really like the look of copper tubes in a PC and am looking at doing a custom loop when I upgrade my PC. There's also the added benefit that copper naturally resists bacteria and fungi.
For a very long time the drinking water in your faucet ran through soldered connections. Even Lead(Pb) solder connections. So it's probably doable.

You might consider using sharkbite style fittings too. These are pushfit connections that don't require soldering. Simpler assembly and similar functionally to standard WC fittings.

A couple of the youtubers have done copper tubes. Linus did one for sure. Thinking about it he also did the whole house water cooling which had many soldered connections. One of the GPU block in the whole home system did foul pretty bad but it's hard to say why that one failed and none of the others did.