Yep, you can pick up a 4TB usb-c SSD for a few hundred dollars/euros. Unless you are moving around 10s/100s of GB routinely, it's not going to be horribly slow. I had a 2TB Samsung taped to my imac for a few years when its fusion drive failed. The 64GB SSD still worked. So I used that for the OS. And the Samsung for the rest. I had Steam installed and X-plane and a few other things. Worked great. This was the 2015 5K imac. Eventually the motherboard died. I still have the SSD.
I've considered getting a mac mini with decently specced CPU/GPU and plenty of RAM and then just attaching a big SSD via thunderbolt. Probably a lot cheaper than maxing out the internal SSD and I don't think it will be that horrible. My main use case would be dealing with photos, maybe X-plane, and some videos. I might buy some games as well but it's not my core use case. It seems the Apple store is slowly filling up with a decent selection of ported games. I gladly pay the Apple tax to never deal with Windows again. I actually have a linux laptop running Steam. The hardware is just really crap and I keep longing for my macbook whenever I have to use it. Actually typing on this thing right now as I'm traveling and I left my work M4 Max mac book at home (it's a bit of a beast to lug around on vacation). The mini would probably be hooked up to a TV so I can watch stuff via Firefox and use a sane ad blocker and UI rather than dealing with whatever crap tastic shit comes with modern smart TVs.
So a reasonably beefy mac mini would basically be my entertainment center and double as a home PC with a ginormous 4K screen. I have considered getting some AMD equivalent with Arch Linux. Still on the fence about that. But either way, external USB-C for storage seems fine.