Anecdotally, I was told that GitHub does not hire from the outside above the senior level. Going from senior to principal engineer is expected to happen inside the company.
Or are there transferable skills that are valuable enough that a transfer could happen at roughly the same comp level?
I don't know but this is an interesting question in terms of human capital and labor mobility.
You have to think about what you want to be doing. I think a strategy that's (company) tribal-knowledge dependent is a risky one, since it's going to be very hard to transfer. Industry-level tribal knowledge can be much more valuable, since you could move to competitors. Technical domain-level knowledge can be even valuable still, but if you're doing more soft-skill architectural guidance and mentoring you might still run into a shortage of companies willing to pay the same premium you're currently getting for that. Some companies may not need it, others may need hands on speed more at the moment. But in that case, they probably do need soft skill technical expertise + management, which at a small, young company is going to be much different than big-co management anyway, so that's an option too.
The bay area does have very big handcuffs, though.