I guess if you’re using this phone as just a Signal machine, I guess you’re fine? It’s when you have different apps and such going on that things get more hairy.
Part of the privacy you get from dumbphones is that they’re so basic, meaning you can’t do much with them and thus pump less data into it. The downside is that if you don’t have a custom ROM, there could be processes going on that you don’t have access to or don’t know what they’re doing. If you use a custom ROM, it could be open source so everyone can see what the phone should be doing. With a dumbphone you still don’t know what that could be.
With that Nokia you’re suggesting, you’re still running Android, so there’s Google Play Services running in the background. It would be like a buying an Android phone with limited functionality, but also potentially limited access to making it private yourself in the settings. In fact, it may be as private as Samsung phone or something, only with this phone you are doing way less on it.
So there’s pros and cons. I think that a Pixel with Graphene or Calyx will be more private than this Nokia phone, but this phone may help you be more private than a regular smartphone because you can’t realistically do much on it. Not a bad option if it works for you.