I do not wish to use Google's Messages anymore. I've come across QKSMS, but I feel like I'm losing some features with their premium service. I've tried looking for the default AOSP app, but they're not available anywhere on GitHub, which makes it really hard to update periodically via Obtanium. I'd love to see apps similar to "Material Files" by zhanghai - which is almost similar to the AOSP File manager. I was also considering Signal, but I don't appreciate the fact that I'm supposed to provide a phone number.
Ps. I'd also appreciate contacts and dialer alternatives.
Try Simple Mobile Tools. I've had mixed experiences with them. Some were great and others were a little too simple for my use cases. But I've mostly heard good things.
ETA: They're available on F-droid and can be updated with Obtainium.
A word of warning about replacing the default SMS app, some manufacturers tie the emergency alerts functionality to it, and if you select another app as the default (as Android insists you do for some reason) it's possible you won't get emergency alerts anymore.
Yes, it hasn't been updated for a long time, I don't understand why, but it can encrypt messages, which is my favorite feature, so I use it all the time.