I havn't updated my phone for years

No,
no need to go back to stock FairPhone OS

Using a FairPhone, no problem,
Using T builds, no problem,

Take a backup and try fist to simply dirty upgrade,
Even if coming from Pie (older than Q), you may be forced to format the /data partition at some time (not for sure)

Go direct to point (8) or (9) here :