Danube cruise choice/ add Prague

If you are looking for a recommendation for a cruise company, we just did a Riviera Danube cruise, and couldn't fault it.
Mostly British passengers, with a sprinkling of Americans.
Most of the Americans we met on shore seemed to be on Viking.
We didn't do an add on, though.

I did a Danube river cruise with Gate 1 that had two extensions. Prague at the start and Romania after Budapest. It was a very positive experience. You could just do the Prague extension either at the beginning or end depending on your direction of travel.

The ships, itinerary, docking locations, etc are all very similar between the river cruise companies. The big difference is price! Do be aware that low water has become an increasingly common issue and could impact your cruise on any line. AMA waterways claims that their ships handle low water best. Not sure if this is really true as they are also struggling at the current time.
Gate 1 River cruises general info
https://www.gate1travel.com/river-cruises/europe-river-cruises

Gate 1 Danube with Prague
https://www.gate1travel.com/tour/danube-river-cruise-12dpbudzpm27.aspx

My brother did a Viking cruise and enjoyed it. They also went to Prague but they did it themselves, They booked a private transfer from the boat to Prague and booked the hotel themselves. He said it was about 40% cheaper than the Viking add on.