Hello Piper28,
What you are doing in right, thats the supported way of upgrade. First you would upgrade the SEPM, then upgrade the clients. SEPM is backward compatible it can manage all the previous versions of 12.X.
Your remote GUPs would still work so do the clients even if you dont upgrade them , this gives you time to plan the upgrade.
GUP is also a client, you upgrade them the same way as your clients. Put all GUPs IP address in a txt file.
Import them during the migration and deployment method.
This document will help you out on this
https://support.symantec.com/en_US/article.tech102907.html