128bitpotato GrapheneOS significantly enhances both security and privacy, as demonstrated in the Usage and FAQ documentation. Many banking apps also perform as expected. You can essentially use GrapheneOS like you would any other Android phone.
Like you said, there is a tradeoff of convenience when privacy is a concern. If you use internet-based services, AI-driven assistants, and similar software, it is difficult if not impossible to ensure privacy. GrapheneOS by itself is very respectful of your privacy, but once you start installing additional applications, you can easily compromise yourself. For example, GrapheneOS won't prevent you from installing Facebook or Instagram, or installing Google Photos and using cloud sync, but all of these activities will directly impact your privacy.
If privacy is the goal, I recommend using offline apps whenever possible, denying network permission to apps whenever plausible, and avoiding services and components delivered by companies known to be invasive.
According to the documentation, restoring the stock OS is just as easy as installing GrapheneOS itself, but I haven't done so myself so I can't speak to it.