Apologies for what might be a stupid question - I saw spree have implemented admin backend in turbo stimulus - will that be ported into solidus or is there a way to use that work in a solidus store?

Spree and Solidus have diverged too much at this point, so that's unlikely. That said, we have been considering what tech we should use to replace backbone in the admin and we may go with the same frameworks. No decision has been made yet.

thank you Jared - is there a proof of concept or road map - as I need to work on a pitch and show what the next 18 months looks like

There's a PR by @kennyadsl here exploring using Hotwire. The roadmap is here.