Yes, lots of Intel and AMD based SBCs have SO-DIMM slots, but you'll have to accept the 3.5" format.

Regarding RISC-V SBCs, there was serious consideration to release the Milk-V Oasis with SG2380 and LPCAMM2. But this didn't work out as the SG2380 was held up by geopolitical issues.

the only one i've seen in a comparably small form factor is the UDOO Bolt https://www.udoo.org/discover-the-udoo-bolt/

but it's an x86 system and pricey, and borderline not an SBC

memory is soldered in most of these smaller systems including the Orange Pi and it's the main price differentiator

RAM is by far the most expensive thing in those systems past the very lowest specs, this one goes from 2GiB to 8GiB AFAICS (amazon page: https://www.amazon.com/Orange-Pi-RV2-Development-Ubuntu24-04... )

the Orange Pi 5 plus comes with 16GiB but the price jumps to US$160+ or 32GiB for US$ 270+

It baffles me that virtually all ARM and RISC-V boards have soldered RAM.

Orange pi 5 plus has 32gb versions. Pricey though

Not really. Most in the ARM space, at least, are soldered on or you need to switch out the entire compute module. Intel ones (not mini-PCs, but industrial gear and things like the LattePanda) also tend to have soldered RAM.