I got my 2018 in 2020 (right when they dropped the prices on flash storage) to upgrade from my 5,1. i7, 32GB RAM added aftermarket, 1TB SSD. With a 6600XT eGPU, it still performs very well, and certainly based on GPU power it's still competitive with the M1.
Right now I figure I've still got at least one more year of service before the per-month cost of the machine has amortized to an acceptable level, but I'm curious to see what the Mac lineup ends up looking at that point when I decide to buy. The Mac Pro will probably be too expensive to be worth it (versus just getting a cheaper computer and upgrading more frequently.) The Mac Studio is basically a better, more connectivity-minded setup than what I've got now, albeit at a higher price than what I bought the mini for (especially once you factor in the upgrades I'll definitely want) and I presume the Mac mini won't change that much from its current template. The question really becomes whether Apple releases an M
x Pro-powered Mini, because that might hit the sweet spot for performance/cost on a machine where you can't really upgrade it.
Hell, unless my work requirements change, I might hang onto it until it's just not getting software support anymore. It's been a great computer (things like the Mojave sleep crash issue aside) and I'll probably still keep it around for old 32 bit apps indefinitely.
My 2018 i5 with 8G is limping toward the finish line. If there isn't something new by March, I am going to have to go with something else like the Mac Studio. It would be overkill for me, but I wouldn't have to worry about something new again for quite a long time.
$50 would get you 16GB of memory and probably give it a pretty big boost (I dunno if the second DIMM is filled, you could get a single 16GB stick at the same cost and end up with 24GB.) Even factoring in the time to upgrade, that's a lot cheaper than a new computer and you'd probably feel like it's an enjoyable experience while you wait.
I was thinking about getting an M1 Mac mini to replace my 2014 Mac mini but now that I have thought about it so long, with the Mac mini due for an update, I am just giving it some more time.
On the plus side, as with all these things, when you do upgrade it'll feel like a quantum jump

I've always found the sizable step up from waiting longer between upgrades more enjoyable, not to mention more economical, than quicker update schedules.