Zammad may tick most of the boxes, though I’m not intimiately familiar with osTickets collaborator fuction. Zammad if others are on To/CC lines then when you “reply all” they stick with the ticket. THough in my install I see that it’s not easy to see who’s on a ticket. I’d be happy to let you be-bop around my home install sometime as a demo.
It has organizations and they can be selectively shared. Though, I think it’s “all users can see all tickets within organization” or “all users can only see their tickets”. Not a “users can see theirs, managers only can see organization”.
It’s a very polished interface, includes SSO integrations, and can be white labeled.
Has email scraping, form submission, and API/webhooks, and some workflow automation funciton.
It also has good agent-side support to have standardized signatures, response templates, and visibility on activity and some OK reporting.
For its price?