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.
How about a DNS-based ad-blocking service? NextDNS is pretty good and not expensive. You should check if you can set custom DNS servers on that Chromebook, though.
I have Yunohost installed on my local network and they have DNS adblocking apps that you can install.
You can also very easily install apps like Owncloud to have your own version of Google Drive.
That probably will not be suitable because the Chromebook could leave the home network.
