CMS for managing borrowed items with family and friends

Are you interested in hosting this for people you already know and trust, like friends and neighbors, or am I misunderstanding your use case?

I've never tried it myself, but maybe see if Snipe-IT would work for your needs?

From their docs intro:

Snipe-IT was made for IT asset management, to enable IT departments to track who has which laptop, when it was purchased, which software licenses and accessories are available, and so on - although we've definitely seen folks using for non-IT asset tracking as well. Oil rigs, theater equipment, even human body parts! (We didn't ask too many questions about that last one.)

Unfortunately it doesn't support OIDC.

There's also the Koha library management system which does support OIDC.

The phrase to use is Tool Library. I don't know of any open source offerings, but this may work. I haven't tried it

https://koha-community.org/

Sounds a lot like a library system.

You could probably use Homebox for this use case. It allows for locations to be configured, attach receipts/maintenance reminders/manuals, etc. Also has OIDC, invite links, and more. I use it to maintain my home inventory of stuff. I think you can also have separate inventories per person/group as well as shared ones within the same instance.

https://sqstorage.net/ could be used for that. Just add storages corresponding to the various locations where the items actually are and add meta information like the owner.

I believe it’s a way away from consumerism if you don’t own everything yourself and share things with people nearby.

Yes, but you break it - you buy it. That seems to be the sticky wicket.

Koillection might fit!

So what's wrong with a calendar and notifications?