(Tailscale cofounder) Many years ago I made a VPN for experimental purposes that kept IPsec’s data plane but abandoned IKE. It’s actually a pretty good arrangement and might be applicable here.
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.
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.