We faced a situation where we had about 4,000 custom Notes apps to migrate, a small team and an extremely limited time. (and varying levels of complexity in each app)
What we found we could do, was "expose" the apps as "Notes Web" apps, and extract much of the UI/UX, and other artifacts - then we wrote conversion utilities.
We were able to get the migration time for simple apps down to the time it took us to demonstrate our framework... (20 minutes per app).
However - quite honestly, the more I saw of Notes at the time, the more I asked "Why migrate?". As the feature-set of early "SharePoint" was not even "at par" with Notes - even now, with the myriad of M365/desktop/app integrations, it is generally "just ok" and does not feel "cohesive".