I do paid media for early stage B2B SaaS. My take: Don’t do PMax. Junk conversions. In platform will look great but there will be no pipeline.
Discovery may be an option.
I would put money in social if by registrations you mean like webinar registrations.
If you mean registrations like, your company has a PLG motion, then I would focus on tightening up search keywords and running cold and retargeting ads on social.
Let me know if you have any questions, happy to answer. 👍