I wish there was a PGP SMS app since the Apple app store does not allow for submitting an SMS app to its store, but the Apple app store does have PGP apps like a PGP keyboard app and PGP encrypt/decrypt apps and PGP is a good standard in my opinion.

Silence is good but the E2EE feature is limited to those who use Silence on Android, so your iPhone friends will not have E2EE. However Silence has not seen much updates which tells me it is discontinued.

QKSMS is a good basic SMS app that I have used before, its FOSS

If you want to get your phone number off of your SIM card and on a "server" you can also check out JMP chat. The cons of this is that it works over XMPP protocol and it is not using encryption between your device and the server, but your phone number is stored on a server and not on your device which enhances your privacy from phone number tracking. You can also SMS and call on other devices with JMP such as tablets and desktop. I hope JMP chat introduces Matrix protocol a an alternative to XMPP and makes a good Matrix client for mobile and desktop.