My 2018 model (which is the same as the "2020" model), has RAM that can be replaced, but it's a little more involved than just popping open a little cover - and the wording around the warranty kind of suggests it's not for "end user" replacement. That being said, tons of people have done it, most without any issues.
The thing with me:
I don't like AIOs, I want to source my own display(s), I want to put the machine where I want to put it, regardless of the location of the display. An extra perk for me, it's portable, like I can drop the Mini into a bag with a small form KB and mouse and just need a display at my destination - not a common thing, but I have done it.
As much as I tinker with hardware, I decided I didn't want any warranty concerns and wanted to add AC+, so I got a refurb with 32GB of RAM OOTB. At the refurb price, the difference in DIY RAM and Apple OEM RAM is about 2-3 hours of billing for me, and this is a "business" machine
The only place the current Intel Mini suffers is in GPU performance, everything is fantastic. I avoided an eGPU as it sounded pretty halfassed, and the initial M1 machine wasn't quite the specs I wanted in RAM and ports, the GPU is excellent, so I'm assuming an M1X based machine with 32GB RAM a kickass GPU and proper port selection is going to be my next machine.