I have been looking this forum now for a while, but can’t comment from first hand user experience point of view. C2 has pretty low/mid range HW, which doesn’t have fingerprint scanner, just FYI. But it seems to have fastest development currently. With Fairphone you will get better HW and with s/OS you will get more polished OS as it is basically android. SailfishOS offers true alternative to android but as you will notice by reading this forum it is not yet as polished as android.
About the apps, there apparently is hiccups with OP app but can be used with workaround Finnish OP-Mobiili - #28 by jovirkku. Don’t know what kind of situation is with Brave, but at least to my understanding Firefox works great. Don’t remember reading anything about Proton Mail but can’t you just link that to any email app? For Bitwarden there is native app called BitSailer BitSailor support thread, I recommend reading experiences in that thread. For youtube there is native app called Microtube.
For others I highly recommend searching answers from this forum. For example there is also native app for HSL called Matkakortti — SailfishOS:Chum, a Sailfish OS software repository by the community. There is native maps app etc. All apps might have less functionalities than android counterparts, some might not work at all. It is a little bit hit or miss, but people have made ton of amazing native apps for SFOS. Jolla is doing their best to offer support but when the apps updates, it might stop working at least for while. Best side of SFOS is that it is true alternative, offers more options to hack or customise it and you own truly the device + they offer long support. Negative side is that it is not as polished and you might find problems with some apps.
This same question have been asked quite often, so I recommend reading them also from this forum. In my opinion C2 is cheap so pretty low risk to test it in Europe but do consider if you are ready for the eventual hiccups.