Ask HN: Best OSS for custom form builder within multi-tenant SaaS application?

I am building a multi-tenant SaaS (CRM) for a small non-profit niche. Custom form building will be a critical piece for my customers. Forms will be used for e.g. event registrations, donations, volunteer coordination, food orders, seat assignments for fundraisers, accepting payment from credit cards/Venmo/ACH/etc. Admins for each tenant will be creating different sets of forms.

I wonder if I can avoid re-inventing the wheel, particularly with regard to an easy-to-use form building UI.

I have been looking at https://github.com/formio, but I wonder what other OSS is out there. Does anyone have experience with this question and/or recommendations?