@Battlegrinder
As a complete beginner who haven't even bought anything yet (actually, given all the advice online that I should focus on the models and lore I like rather than how newbie-friendly the gameplay is, I'm heavily leaning Tau at the moment), would you say I should ignore all that stuff about swapping weapons for now, at least until I get a few models assembled and painted in my figurative resume? Magnetizing sounds like a verh advanced technique.
That somewhat depends on faction. If you're doing marines, you can probably avoid having to magnetize anything for a while, until you have a more solid understanding of painting and modeling.
If you're doing Tau, you might not have that luxury, particularly if you go heavy on crisis suits. You'll want to build those with swappable weapons and equipment, it will be prohibitively expensive to build up a collection of crisis suits with all the different configurations you'll use fixed in place.
People are correct that you should pick a faction that you enjoy the models and lore for, you're going to spend far more time building and painting then playing so it's important you enjoy that side of the hobby. That said, I would not make the your solo criteria, unless you really, really, really love the look or story of something and don't care about performance (and speaking as someone that owns an Arvus Lighter*, I completely understand and support that mindset), how well it does on the tabletop is a factor that's worth considering.
I don't know how Tau do these days, I've heard they're in a bad spot because 9th edition rules just don't favor them, but I haven't faced a real Tau army yet so I can't speak from my own experience on that either.