We also have done a mixture of
DCL and independent excursions over the years, but now we almost exclusively book independently unless what we want to do can only be booked through DCL. In our experience the independent tours have been at worst just as good, and usually much, much better than anything booked through the cruise line. While tour companies may be looking for good feedback from DCL, you can better believe they are looking for good feedback from their independent customers as well. These companies live and die by online reviews. They are trying their best to make all of their customers happy, not just the ones booked through a certain cruise line.
We now try to avoid the cattle call tours if at all possible and go private. We do what we want, when we want on our own timetable - and have had some terrific guides. We also had an interesting discussion with one of those guides regarding DCL:
When we were in Kirkwall, Scotland we booked a private tour with the company most people recommended for this very small island. We were fortunate enough to have the owner of the company as our personal guide for the day. As the day went on, it was plain to see through her interactions with people we met, places we had access to (that no one on a DCL tour saw BTW), that her company pretty much ran the tourism industry in Kirkwall. This was the first time that DCL had docked there and she mentioned that several months ahead of time, DCL sent in representatives for a meeting with all of the tour companies in the area. They outlined how they wanted their guests treated, what they expected, etc. According to our guide, the tour operators were mostly offended. They took it as Disney trying to tell them how to do the job they very well already knew how to do and had been doing for years. Now, from another standpoint, yes, DCL was looking out for their passengers. But as our guide basically put it - they have been taking care of their tourists long before Disney got there and will continue to do so long after they leave. It was an interesting conversation.
Another interesting tidbit our guide told us - there was a group of imagineers on the ship and they had booked a private tour through her company as well. Now, her company was also running several bus tours for the cruise line, but the Disney employees chose to book privately with her!
OP - I know we've gone off topic, sorry, but felt the need to chime in on the private tour experience. Hope you enjoy Blue Lagoon however you end up doing it. We did this years ago (through Disney - at that time there really was very little difference in price and the experience was going to the be same either way) and really enjoyed it.