To be honest, I really don't enjoy the tech progression and base management aspects of XCom. All I want to play is the tactical missions.
Then Jagged Alliance might be for you. Come to think of it, it is much more focused on the boots-on-the-ground tactical gameplay than the strategic meta. Perhaps that's actually why I prefer it as well!
What's the difference between the Steam deck vs the Nintendo Switch
The Deck is much bigger. While that makes it less portable, it is also much more comfortable to hold. I always found the Switch somewhat cramped to hold, not so with the Deck. It's really astonishingly well-designed hardware. The parts that get hot are far from the grips, and everything is layed out in a very ergonomic fashion, at least for my hands. And, crucially, it has the same instant-off, instant-resume functionality of the Switch.
Otherwise, it is "just" a PC. You play PC games, with PC configurability, and PC prices. There are some clever additions for making PC games playable on a handheld, such as an always-available on-screen touch keyboard and the two touch pads for (surprisingly usable) mouse input. The input system is crazy good. You can define macros, open menus, define modes, use gyros, adjust curves... and most importantly, just import someone else's efforts to make, say, an old game playable. Even thoroughly mouse-based games like Command and Conquer or CRPGs are eminently playable.
And being a PC, you can trivially expand storage with a micro-SD card, just plug in an external USB-C drive, or in fact replace the built-in hard drive. (You can even drop into Desktop mode, plug in mouse and keyboard, and do your taxes or whatever. But if that's your jam, just get a laptop, frankly.)
The point about the catalog is huge, though: since it's a PC, you are not limited to one vendor's current offerings, but have all the history of video games available, including extremely well-integrated and performant emulators for anything up to PS3/XBox360/Switch. Of course Steam is the best-integrated vendor, but GOG and Epic are pretty good these days as well, with the Heroic Launcher.
The main limitation is that it runs Linux under the hood. Not that you'd notice that most of the time, but there is the odd game that doesn't run on Linux, usually due to some Anti-Cheat incompatibility. Microsoft Store games can be difficult as well. You can actually just install Windows if you'd like, but then you give up a lot of what makes the Deck unique.
Being a handheld, its power is somewhat limited. The low-resolution screen and modern frame generation techniques help, but expect 30 FPS on recent AAA games on mid/low settings. Older or less demanding games are no problem of course. Battery life on those titles is limited, on the order of two or three hours. Less demanding titles run much longer of course. But I've played through e.g. Death Stranding on mid settings without issue.
Speaking of Jagged Alliance, I have played a few sessions on the Deck, and it works fine. Surprisingly so, actually. But, for this game in particular, I do prefer a bigger screen and a keyboard and mouse, so I played most of it on my Desktop. I the past year, that has been the exception, though. I'll go on vacation next week, and then I'll continue my conquest of Jagged Alliance on the Deck.
I'm still waiting for Heretic & Heretic II to be playable on the X-Box, being able to turn your opponents into chickens was a blast back in the day. 
