Has anyone had any success getting an IPSec VPN setup compatible with the built in Windows 10 client ?
The IPsec is fairly straightforward to get configured on the Windows client. But based on the settings on OPNSense one might have to use Powershell to tweak the Windows client to match the configurations on the OPNSense server
Use IKEv2 if you can. v1 can be a world of pain.
https://docs.opnsense.org/manual/how-tos/sslvpn_client.html
Cheers,
Franco