Looking back on the project, there were a few lessons learned and hopefully we can help you with avoiding these mistakes

Estimations

When starting the migration project, we estimated that it would take longer than 1 sprint (we do 2 weeks) to fully migrate the application. A big investment, but worth it in the long run.
It turned out to actually be a lot shorter than that; it took 1 FTE about 1 week + some extra for reviews to eventually migrate the project.
The fact that we could upgrade to Capacitor 4 & 5 without having to upgrade all plugins, made progress a lot
faster.

The biggest trouble we got was from the FileReader, where we had a ZoneJS instance and a Capacitor instance. This was very tough to find out, but at one point the dots got connected and we found a solution to use.

Could we have done it better?

Probably, but not by a lot. When we started the project the target was to finish. We might have been better off with some more Proof of Concepts and prototypes to see how the migration would go. That would have given us a better idea and estimation on how to tackle it fully.

But in the end we are very happy with how the migration has gone and we will be better focused on future releases so we’ll have an easier time upgrading again!