My experience with the Bluetooth connection on both the steam deck and a miniPC I installed HaloISO on (Arch Linux/SteamOS, with hardware restrictions removed) wasn’t that great. My environment could be a factor.
I sit about 3-3.5 meters from the TV and Steam Deck/MiniPC. I have used DualShock, Xbox One and Series X|S controllers. The TV is against a wall and the ‘consoles’ sit on top of a dresser beneath it… No shelves or enclosures, other than a thick sheet of glass on top.
In the case of the Steam Deck, I keep a solid connection if I’m pointing the controller towards it. I do not notice one controller being any more reliable than the others. A slight restriction, but essentially how I played like 80 hrs of Midnight Suns. In the rare event I’m in windows on the deck, I’m generally not gaming, but it will use an Xbox wireless adapter plugged into its dock if I try.
The MiniPC is hampered by its construction, with diminished wifi and Bluetooth signals. I get around the wifi by hardwiring it to my network. In Windows, it uses the Xbox wireless adapter without issue. In SteamOS, there are no straight-forward linux drivers for the adapter, so I have to plug the controller in with a long USB cable if I want to game there. For this reason, most games are played in windows, through Steam or PC Game Pass. Unfortunate, as the miniPC was intended to be primarily a steam machine.
When upgrading your graphics card, which is all you can really do, other than upgrade RAM if your mobo allows it, physical dimensions and power requirements are the two factors. It may fit in the chassis, but if the card’s power requirements exceed what your power supply provides, you’ll be screwed.