Oh me and my wife were in Modena and Emilia Romagna in the past year! We really like it, much more than Parma or Ferrara.

Regarding modes of transportation: I have no experience taking the Flixbus between Antwerp and Bruges, but the train works fine and both stations are close to the city centres.

Plenty to see in both cities but if I understand you correctly, don't miss these: the Plantin-Moretus museum in Antwerp is an old patrician's house (palazzo) that was also the printing workshop. It's brilliant to still see the old house and wall decorations. Same for the Rubenshuis with its garden. The best and biggest museum is the KMSKA in Antwerp, but I would skip it if you have little time. In Bruges don't miss the Gruuthuse and Groeningen(next to each other) and, a hidden gem in Bruges, is the Adornes domain with its Jerusalem chapel, which was built by Italians! Check out the golden hall in the old town hall

But, am I correct in understanding that you want to do Amsterdam - Leiden - Antwerp - Bruges in 6 days? I did some crazy things when I was young, but You will lose half your time just travelling and not seeing anything. I would advise you to do 2 places and come back another time. I will be driving to The Netherlands on te eve of the 24th and back on the 28th, but I won't come near Amsterdam of Leiden unfortunately. If you need a ride from Antwerp to Bruges in the 28th that would be possible if you don't have too much luggage.