crypto ikev2 proposal DXC
integrity sha256 sha512
group 14

Then I’ll prioritize it with:

crypto ikev2 policy 5
proposal DXC

crypto ikev2 policy 10
proposal proposal1

IOS evaluates policies lowest → highest, so DXC will be tried first. If a peer only supports 20/21 it will fall through to proposal1, so existing tunnels shouldn’t break. The only change is some peers that support DH14 may negotiate that instead on rekey.