Unlocking the bootloader and leaving it unlocked would give you TWRP as a backup option.
The data partition will be encrypted nonetheless, so simple loss or theft of the phone shouldn’t lead to access to the data once you have provided a decryption method (= your lock screen password/PIN/pattern). Anybody else but you would have to factory reset to use the phone.
This leaves a scenario where somebody would target you specifically by stealing your phone, installing some program to grab and transmit your decryption method once you unlock, and getting the compromised phone back to you, all without you noticing, to then let you provide the key to the data.
How risky this would be for your circumstances is for you to assess yourself.
Being able to use TWRP sure is tempting, though.
Rooting the phone should enable some popular backup options like e.g. Titanium Backup, but I don’t have experience with this. There’s enough about rooting here or over at the Fairphone forum, just search for root FP3.
Another option would be to just wait for /e/'s integration of Seedvault to see what this can do then.