How to progress as PhD student with nonchalant supervisor
Ideally you need to find someone else who can unofficially supervise you. While such a hands-off style of supervision can benefit a student in the long run (by essentially forcing you to work independently and follow up your own ideas), it's not great when you run into the typical day-to-day problems of research. In this situation, as you have realised, you need someone who you can reliably talk ideas and problems through with on a regular basis.

Rather than contacting other academics, are there any postdocs in your department who you could talk to? Or even perhaps a postdoc from another university who you've met at a workshop or conference? They would be the best sort of person to try and start collaborating with. Postdocs generally have more time for research and if they're keen to stay in academia would probably welcome the opportunity to help supervise a student. The relationship doesn't have to be formal in any way -- you can just start by sending an email. Be clear and upfront with why you are contacting them specifically, so it's obvious you're not just pinging off emails to everyone you can think of in a scatter-gun approach. Ask if they have time for a video meeting, although bear in mind that at the moment, everyone is over-saturated with calls and telecons.

The best way to then move from conversation to collaboration would be to have a concrete idea or question to answer, and preliminary results if possible. This way, the potential collaborator can see you're committed to the project and have an idea of where it's going. Make it clear that you'd like them to be involved in the paper that you're planning to write (or, if you have notes or a preliminary draft, share that too). As things progress, you can then ask for the detailed feedback that you're not currently getting from your official supervisor.

I speak directly from my own personal experience: my supervisor, like yours, is very busy, although we do fortunately have regular meetings. However, for technical matters on which he is not an expert, I have a postdoc collaborator who I can talk to and ask for help.