Yes, but to get something nearly as good as 365 would realistically cost 100x as much as just buying from Microsoft.

Who would you even hire to do this? A big consulting firm known for delivering poor quality software, or an unproven startup? What kind of a process could you use to make such a selection in a way that would ensure a good outcome?

It’s the same as the old libreoffice vs. MS office debate. Yeah, you can download libreoffice for free. It sucks. What sucks even more though is how much money you will spend on support, training and all the inevitable productivity losses associated with weird software that approximately nobody you hire will have experience with.

In theory, yes, it could be...

But these are "European Tenders", which in practice usually translates to: race-to-the-bottom. Unless the tender was phrased specifically, from its very first inception, to aim at some polical goal - like open source, sovereignty, innovation, inclusiveness, etc.