Not totally relevant for the transparency aspect, but that license is just “source available” and not open source (as in OSI definition) and definitely not free software. Of course this is much better than being closed source, but legally it’s still proprietary. If you care about security, privacy and also about the ideology aspect of open source, i.e. the freedom that you get as a user, then MEGA unfortunately is clearly below the other solutions mentioned.
So, with this information in mind, what would your preference order be, between Proton, Filen & Mega?
Price-wise, it’s hard to beat MEGA. Especially in your situation, where you cannot take advantage of anything else offered by Proton, as you’re already covered for the time being. So my preference would be, if I can afford it: 1) Proton, 2) Filen, 3) MEGA. And if I would be looking at saving money, then my preference would be: 1) Filen, 2) MEGA, 3) Proton. I think that all these solutions are secure enough for general usage if you’re not in a special situation, like if you’re a politically exposed person or have close ties to someone like that etc.
Would iCloud ADP also figure in your recommendations here?
Security-wise, iCloud with ADP should be pretty good. Apple engineers are not stupid. What I personally dislike the most about iCloud is that, in my past experience, I sometimes came across weird glitches where certain services stopped working properly, only to resume working at some point in time, or just staying bugged indefinitely. And Apple Support, regardless of how many times you call them and get transferred through all kind of departments, ultimately not being very helpful (but very friendly and attentive, gotta give them that). I never lost data, but these glitches coupled with the fact that iCloud is really only usable on Apple operating systems (trust me, iOS/macOS etc. are great and all, but I just want/need to use all kinds of different OSes and machines), made it uninteresting for me.
I have also used stuff like Cryptomator, Plasma Vaults etc. and well, it works and it’s good but it isn’t seamless enough for me to put all my stuff in there. I only use it for some extra protection on the most sensitive data.
That being said, for long-time backups I do use Borg with https://www.borgbase.com and https://rsync.net – I consider the solutions that you mentioned (Filen, Proton Drive, MEGA, iCloud Drive etc.) as more of a syncing utility so that I always have my documents at my peril regardless of where I am and what device I am using. But because I’m generally actively logged into a service like that, malware could wreak havoc, or I could mistakenly delete stuff, or the service provider could have issues, so an actual append-only backup every so often is still important to me.