The requested feature "Rollup summary for those custom modules linked via a lookup field" is currently in early access.
With this feature, you'll also have the flexibility to select any field from a system-defined module's related list or a Custom module's related list.
Additionally, in the linked announcement post, we have provided a form where you can request Early Access.
I'm locking this post and we encourage everyone to follow the announcement post below to request early access and share your feedback.