For what it’s worth, I do think OCIS is worthy of switching to if you don’t make use of all of the various apps Nextcloud can do. OCIS can hook into an online office provider, but doesn’t do much more than just the cloud storage as of right now.
That said, the cloud storage and UX performance is night and day between Nextcloud/Owncloud and OCIS. If you’re using a S3 provider as a storage backend, then you only need to ensure backups for the S3 objects and the small metadata volume the OCIS container needs in order to ensure file integrity.
Another thing to note about OCIS: it provides no at-rest encryption module unlike Nextcloud. If that’s important to your use case, either stick with Nextcloud or you will need to figure out how to roll your own.
I know that OCIS does intend to bring more features into the stack eventually (CalDAV, CardDAV, etc.). As it stands currently though, OCIS isn’t a behemoth that Nextcloud/Owncloud are, and the architecture, maintenance is more straightforward overall.
As for open-source: OCIS released and has still remained under Apache 2.0 for its entire lifespan thus far. If you don’t trust Owncloud over the drama that created Nextcloud, then I guess remain wary? Otherwise OCIS looks fine to use.
From my experience with a modern Thinkpad (A485); nothing if not outright inferior. The trackpoints on them are pretty terrible compared to classic IBM-era thinkpads (10-20hz polling rate, abysmal velocity curve). The physical durability of the machine might be above-average for business laptops, but the chance of the hardware failing in some major way within warranty seems to be quite high (among other replacement parts, I had 4-5 mainboard replacements done under warranty). The cooling solution on the Thinkpad I used to use was also a fair bit inadequate, and would lead to severe thermal throttling of the mid-range APU. Honestly between the reliability and torturous process to even buy a new Thinkpad from Lenovo, I just wouldn’t bother.