There are only a few to consider. /e/os if you want it easy or DivestOS if you want it most secure and private. All the other possibilities have disadvantages compared to these.
Please be aware that you should buy FP5 as FP4 has huge hardware issues and the support is a dissapointment. And yes, /e/ is available for FP5 (but not via easy installer, but it's not hard to flash it yourself)
/e/ is announced as 'degoogled' but that's not 100 % true (and not nearly as well). For example MicroG connects to Google as well as connectivity backup check. Patch level is far behind AOSP. The App Lounge uses clean APK for some apps which is very risky. Communication is a problem and they do not react like they should for example when Mike Kuketz analysed /e/ and found several problems.
The community is huge and they support many devices.
DivestOS is better in most points but is managed by one person alone. MicroG is not included by default (if you need it) and multi sim support is a problem.