Thanks for pointing this out, I did read that.
However, IF s.o. with access rights can also download and thus re-share my pics (standard procedure, I'd) say, there is no increased security by a web drive solution in comparison to e2e encryption. In fact, screenshotting would also need to be disabled for the pics to be more secure.
Other than that, having them in a Threema or Signal TL is as secure as the least protected smartphone in this chain. No PW'll change that.