What is commonly called IPsec is actually two separate protocols, IPSec itself and ISAKMP/IKE for key management.

IPSec is somewhat similar to how wireguard work actually, it relies on IPs and static encryption keys. Not too hard to configured, see for example the manual keying documentation of slackware: https://book.huihoo.com/slackware-linux-basics/html/ipsec.ht...

ISAKMP/IKE is then used on top to manage the IPsec keys and parameters. This is where a lot of the complexity comes in, tons of parameters, modes, etc. etc.

So if all you want is to secure communication between two IPs and can securely exchange key material out of bands, manually keyed IPsec is not very complicated.