Okay, I took jimp's advice, and after some struggling with syntax, I was able to get past the NO_PROP error (to run into a different error right behind it).

Anyway, to help someone else with the NO_PROP error, I'll document what I did.

I looked in the /var/etc/ipsec/swanctl.conf file on the Netgate 4100 and found these two lines: proposals = aes256-sha256-modp1024 esp_proposals = aes256-sha1,aes256-sha256,aes256-sha384,aes256-sha512

Note that the syntax is very different from what was shown in the log file such as "AES_CBC_128".

I copied these into the corresponding fields in the network-manager-strongswan VPN settings.

On Ubuntu 22.04, it is in this location:

VPN Settings > Identity tab > Algorithms at the bottom

Check the box "Enable custom algorithm proposals"

In the IKE text input, I put:

aes256-sha256-modp1024

In the ESP text input, I put:

aes256-sha1;aes256-sha256;aes256-sha384;aes256-sha512

NOTE THAT THE COMMAS WERE REPLACED WITH SEMICOLONS! This caused me a bit of frustration until I accidentally mouse-overed the input label and saw that it said the list must be semi-colon-separated.

Anyway, with these changes, I now no longer get the NO_PROP error.

Now, I get a missing public key on the SSL certificate. If I can't solve that, I'll start a new thread.

Thanks, @jimp !