Hi True and Free. Apple iCloud just went to E2E (end to end) encryption but not on everything. Found on their website they don’t have E2E on mail, contacts and calendar. That may not matter to you if just storing docs.
I don’t use Microsoft products so don’t know about MS One Drive. That said, you have to ask yourself do you trust either of those companies?
Another option that you may want to look into is Proton Drive. It is E2E on everything and of course is part of the Proton ecosystem that Glenn recommends. I have not used it and believe they are still developing some of the features that will make it easier to use. In addition, their best pricing comes around Black Friday and usually ends by the end of that given year. That said, you can sign up for a free account to at least try it but it only gives you 1Gb so you won’t be able to store a tremendous amount of large files. But at least you can test drive it for free and could mock up some dummy files before you jump in with both feet.
One other option would be to buy an external storage drive that can be encrypted (or not, depends on what you desire). That way you have total control over everything. One thing to keep in mind is where would you store it? If at home and God forbid there was a fire, then you may not have any files you could recover (computer or external drive). Just something to think about if you were to choose this option.
I hope this helps a little.