EDIT: Apologies. Updated with a link to what gorhill REALLY said:
Manifest v2 uBO will not be automatically replaced by Manifest v3 uBOL[ight]. uBOL is too different from uBO for it to silently replace uBO – you will have to explicitly make a choice as to which extension should replace uBO according to your own prerogatives.
Ultimately whether uBOL is an acceptable alternative to uBO is up to you, it’s not a choice that will be made for you.
Will development of uBO continue? Yes, there are other browsers which are not deprecating Manifest v2, e.g. Firefox.
Multi-account containers?
Those don’t exactly do what I want – which is clear separation of different work accounts. I’m not sure if this is what I need though, so perhaps I should take another look at what you suggested.
In firefox, type “about:profiles” in the search bar and press enter. (Please note that this shouldnt open your search engine) This will open a menu in that tab that claims: “This page helps you to manage your profiles. Each profile is a separate world which contains separate history, bookmarks, settings and add-ons.” I hope this is close enough to the solution youre after!