I basically stopped buying SBCs several years ago, are there any SoC platforms that have mainline Linux support these days? Or is x86 still the way to go?
Libre Computer makes a range of SBCs and supports mainline Linux. I use their products, and I'm very happy with them.
"Most ARM single board computers depend on proprietary binary blobs to boot or ship with outdated vendor kernels that are never upstreamed and quickly abandoned. Libre Computer takes a different approach: we fund and contribute to the mainline Linux kernel and U-Boot directly, ensuring our platforms run on upstream open-source software with minimal proprietary firmware."
RK3588 is well supported right now and is present in many SBCs
The Radxa Orion O6 is a really nice ARMv9.2 ITX board, and supports UEFI boot. Installation of Debian trixie using Debian's vanilla installation media went flawlessly, and it's been running fine for 6 months now.