Offload the majority of work by asking sales teams for each service to define a lead, i.e. the business is in X industry, with X employees, they’ve engaged with X pieces of content and there is a valid email, phone number, first and last name.
Then the other scoring metrics can come from existing customers, if 60 % converted from organic sources, bump up the score for leads coming in that way, if 80% of customers are from certain geo-locations bump up points for those areas, and on the flip side, look at lost deals, or leads that are not qualifying and add negative points to leads that share similar attributes.
I agree though, the whole process is complex and very manual to then create and manage on HubSpot. Worth it though, even if the scoring is used as a guide.
I also implemented a feedback process, for leads that were bumped up but then rejected, ‘tell us why they were no good’ and that allowed me to keep updating the scoring matrix. That said, I only implemented it for one scoring property, it would be more complex to implement that for multiple.
Good luck!