IPsec is a terrible fit. Tailscale uses a control plane for peer & key distribution and renders almost all of the complexity in the IPsec protocol useless.
My implementation relied on the extremely finicky Linux kernel IPsec and would never have worked in userspace macOS or windows. But yes, a Tailscale control plane with a per-node switchable data plane, one of which is IPsec with PQC, is a real option that would work.
By locking down the control plane it becomes incompatible with classic IPsec, but also avoids most of the security gotchas that plagued IPsec.
It could have worked in macOS! It implements RFC-2367 (PF_KEY socket type) and I even made it work on my laptop.
I traced the SA database operations on Windows, and it could have been done. Probably. But I abandoned that to preserve my sanity.