As suggested by @aben at IM, I experimented with KBL framebuffer settings on my Skylake/HD520 Latitude E7270. I also updated all add-on kexts to latest versions (Lily & PlugIns) and replaced good old ACPIBatteryManager + FakeSMC by their VirtualSMC equivalent.
This allowed me to boot Ventura b1, complete 1st boot setup and reach desktop; the laptop still freezes shortly afterwards so something's still iffy but at least I can boot and with apparent full graphics acceleration:
I don't know about older generations but there seems to be hope for Skylake laptops.