Itme
I fit such "simple citizen" description without threat model per se, except that everyone is a target of surveillance capitalism.
Like Blastoidea said, Big Tech is keen on gathering every bit and morsel of everything you do online and can infer a tremendous amount of what you do offline. This data, however irrelevant it may seem to an average voting minion, is worth real money to our overlords (government, BT).
Further, this data is used much of the time against the interests of those very same citizens, whether to herd them like sheep (via "incentives", PC speech), coerce them into obedience (digital passports/money/ID/passes/geofencing/cancelling), profit from them by use of psychology (advertising/influencers), etc., etc. Your own example (re: Signal) demonstrates this very issue.
So, to answer your Q's:
- Graphene is absolutely the best, most secure option available to us, mere "simple citizens".
- my life is absolutely better for having it. Two of the biggest benefits for me are: I spend less time on my phone and I tangibly reclaimed some of the control over my life that we have ceded as a society.
- a major reason you should use it, is specifically to counter your example. Once you're on GOS, it's one less person on that side and one more person on this side. Your government can't jail those folks if majority of citizens are on GOS/Signal/etc also.
I highly encourage you to make the switch. Drawbacks are very few, cost to entry (considering the benefits or not) Is low, falling back is easy, but... the potential upside is enormous.
If you choose not to adopt Graphene - because you're a free, thinking person capable of making your own decisions - I would still strongly encourage you (for the very same reasons): to use Linux et al. for your computing needs; to work on distancing yourself from MAGFA (towards FLOSS); to adopt E2EE services; to support these communities; to persuade others to see the light, for the common good of all.
Hope this helps!