The deck shines with its default OS, and while installing other OSs is possible, it's probably not gonna be a great time. Though you may not need to, as the deck's desktop mode is pretty capable for light browsing/working. Especially now with the addition of the nix package manager.

As for the dock stuff, you're not wrong, but a dongle would work just well as a dock, so if you want to wall mount it, you just need to leave enough space to hang a dongle to it.

Overall, a beelink would probably work best, and give you fewer headaches if a desktop is what you're looking for, but the deck has the portability/handheld factor. It's really gonna be up to your needs, which one is better.

I had to look up beelink because I had never heard of them. They look cool.

It sounds like the decision is:

Do I get a gaming device that can do light computing?

Or

Do I get a light computing device that can do gaming?

As a steam deck owner, it would probably get by doing the things you mentioned and it would probably be a more stable/easy gaming experience. But I personally wouldnt want to use it as a computer. As soon as you want to do anything non-super-basic it's gonna be annoying since it's an immutable distro and not set up the way most Linux distros are. A dock would solve the port issues. But You also said you are going to switch to Ubuntu and at that point...

The beelink would give you a little more flexibility and if they games you play already run on Linux, then it's gonna be pretty easy to get it working (assuming beelinks hardware is all 100% Linux capable.) I'd go with that. The only benefit of a steamedeck is steamOS and the backing of valve to make sure it's going to work and the portability. You plan on getting rid of the first benefit and don't seem to care about the second.

If I were you, I'd go beelink.

I have both. The deck is not going to be great for anything PC. It'll be great for optimized gaming. Period.

I have actually had a few Beelinks. I immediately put linux on them, as I can't stand using windows. For linux, they work great. Mine is my main rig / torrent server / Jellyfin server. It works great for all of those.

There are different Beelinks, they do not all perform as well as others. I went the AMD route, as they play nicely with linux. The intergrated AMD gpu + cpu are also supposed to have a synergy (not sure the industry term for this) that intel + nvidia do not.

The Beelink, when not taxed, can spool down to less than 10 watts, which is incredible. The fan is not loud, even when taxed. I honestly don't even notice it ever, even a foot away.

I found the Ser5 wasn't capable of running even simple games well. The ser6 most certainly does. I tried both Windows and Linux, and the 6 was better. IF you can afford the Ser7, I'd say go for that. There is also the pricey GTR series, which is their premiums.

Also, consider Minisforum. They've got the best specs of any miniPCs, but pricey as fuck.

The Deck would be fine for what you want, but the Beelink more capable. The Deck would be more portable and therefore useful in that capacity, but it's all subjective. If you don't care about the screen, get a refurbished first-gen for a discount, or take your chances and find a used one on eBay. I've okay things about the ROG, but SteamOS makes Deck a no-brainer in my mind.

I've been using a Steam Deck as my only PC for almost a year now, for work (graphics design, web dev, illustration, some Blender) as well as play ofc. Aside from my suboptimal dock options (Valve doesn't sell any hardware in my market) It's been a very smooth experience, and I've not even had to disable immutability at any point.

I would like to support the point that Game Mode is one of the most important features the Deck has, and losing out on it by installing Ubuntu feels like a loss.

But I would also like to note that Steam OS now has Distrobox built in: for most use-cases you can just set up all the software you need inside an Ubuntu container without much hassle.

Ultimately though, the form factor is the main difference. If I only needed to keep it docked all the time a Deck would not make much sense. But I love shifting to my bed after a workday and playing anything and everything I would have needed to sit at a desk to do before!

The Steam Deck only has a single USB-C port on it, so you would need the dock for USB-A ports. The good news is that the existing USB-C port is on the top of the Deck and the dock connects via a short cable. You can use the dock as a stand but it's not required.

https://www.steamdeck.com/en/dock