Is there a way / best practice to group SSB identities together?
Yes! SSB is fundamentally cryptographically signed JSON in a hashchain (or blockchain). Nostr is just cryptographically signed JSON. Because SSB posts are strictly ordered you can't add to the same hashchain from two separate devices because it forks the chain. I wrote more about it here: https://mattlorentz.com/weblog/2023/01/18/nostr-v-ssb.html
Discussion
Not really, that’s part of the problem. What folks do is they link to their other identities in their bio… there is not accepted cross signing of keys yet. There’s a proposed feature called fusion identities and a new feed format called metafeeds to solve that but it’s been very slow going.
I saw your post on planetary dot social that you are winding it down. I suppose it was not ever possible to have read only accounts on SSB (like they do here on Nostr). Read only accounts can’t fork the blockchain.
We do basically have read only clients. Look at planetary.name as an example.