Migrate from El Capitan (10.11) to Catalina (10.15)
I ended up not migrating the whole machine. Since Catalina supports only 64-bit apps, it makes more sense to re-install one by one. Since most of my files (data) are synced online, no need to move them over either: I can find them back using a cloud storage solution (iCloud, Dropbox, OneDrive or pCloud).

The most important thing I needed to migrate over though that was not synced online was my mail archive: multiple mailboxes sitting on the machine itself (i.e. not on any active account such as Exchange or Gmail). The solution for this turned out to be simple: 

1) El Capitan was cloned on an external harddrive
2) External HD was connected to the new machine running Catalina
3) On Catalina, within the Mail App > Import Mailboxes... > choose “Apple Mail” continue > navigate to the external HD and find user/library/mail
4) Import

To improve management of archived email, I’d recommend using a third-party software such as MailSteward or EagleFiler.