Having only done one cruise I am probably not the best to offer advice, Simon.

We don't look at a cruise as being the obective, but choose to consider one when it seems like the best (or in some cases, only) way to see or experience something. That was why we did the Alaska cruise from Vancouver a couple of years ago - only way to see the Alaskan glaciers. We chose Holland America for this for being a smaller ship (around 1400 pax) & having a reputation of attracting a "more mature" audience but not super formal ( I don't do "dressing up" )- no party ship ! And it went a lot better than I could have expected. The crew were great, the food was great, decent weather for the most part meant that we go to see a lot - Glacier Bay in particular was a spectacular day. Enjoyable way to end the day in the observation bar at the front of the ship cruising down the Inside Passage watching whales jumping. Doing the St.Laurence Seaway this time, starting from Montreal & ending in Boston with various stops along the way, also with Holland America and actually on the same ship that has relocated from Alaska this season. Initially looked to doing the US Great Lakes, but they were eye-wateringly expensive (£10k each minimum for 14 nights + getting there and back).

The Fiords trip is( IIRC) Ambassador Line from Dundee to get most time in the Fiords - other UK departures spend more time at sea getting to and from Norway, so get to fewer places than departing from Dundee.

No epxerience of European river cruises, other than looking at the prices & thinking "how much" !
As these don't do anything I have not been able to do by bike, car & MH over the years they don't have much appeal for me. Be aware that they can be adversly affected by higher (can't get under bridges) or lower (grounding risk) than expected river levels & that there are industrial areas along most rivers at some point - the "Romantic" Middle Rhine is really quite built-up and busy. That said, it is an area that we seem to gravitate towards at some point on a German tour, so it can't be all bad !

We booked through the local Hayes Travel branch who sorted a full itinery of getting there & back, hotels before and after the cruise + the cruise itself. IIRC they use an outfit called First Class Holidays to arrange this. Maybe not the cheapest way do do things, but the bespoke Western Canada /Alaska trip they organised last time went very well for the most part. The couple of niggles we had were communicated to them on our return (mainly car hire issues + one hotel having changed hands) resulted in a reasonable and proprtionate refund by the parties concerned. Prior to booking Mrs B had done a lot of research into the options and routes beforehand & we pretty well told the agent exactly what we wanted when we went in. May have paid a bit more for air fares as I won't use British Airways if there is any possible alternative (the tour planner's airline of choice) & IIRC KLM was more expensive.

I have been told by other, more seasoned, cruisers that booking direct may not be the cheapest way to buy a crusie, as the agents get all sorts of incentives to offer that don't come up if you book direct.

HTH

Nigel B.