I started from the same point of you (Samsung phone → Fairphone and google/Samsung services → /e/os and murena).
First point: No problem. I use Spotify as well and it works without problems. One noticeable thing is that group listening sometimes doesnt work (one reason is the Spotify tracker which has to be enabled) but it might also be that Spotify is buggy. Which wouldnt be a surprise as this feature specifically is buggy on Samsung phones as well.
So yeah, works in the regard you asked without any problem.s
For other paid apps like Threema I downloaded them seperatly and update them within the app.
Second question: At least for steam it was no issue for me in the past altough I switched to alternative 2FA-Apps like Aegis (highly recommend!) as they are open source, allow for easy encrypted backups and are centraliced on my phone.
Banking apps are different for every bank. I suggest looking into this thread and search for your bank: [LIST] Banking Apps on /e/OS
Update Cycle is every couple of weeks as far as I remember, might be a bit different nowadays. I stopped updating rigorously since some updates the battery life suffered a bit. Now I update if there are big security updates or when the next updates arrives. For example when 1.5 is released I installed 1.4 since I then had time to read all the user reviews for the 1.4 update. If you choose to always stay up to date I suggest you wait a couple of days until there are a couple of reviews in the update feedback thread.
Last point: In my experience Samsung had ways less features (I think they disabled a lot of them for some reason). But I also didnt look to much into hacking android. I just use stock /e/ and am happy with it for my use case (daily use, watching videos, listening to music, texting and phone calls).
About the battery life: Maybe it would be a good Idea to buy a separate FP Battery and take it with you in a safe case? Sadly I have not yet found a battery-case but it should be easily producible via a 3D-Printer.
Hope that helped, sorry for the rough English 
Edit: My FP4 lasts for about 1.5 to 2 days. 3 Days might be possible if you use it less than me. No Idea about the FP5 battery tho.