Just my opinion, may not be of much value, and should be taken at that level. I am not a believer in "the sky is falling, the sky is falling" scenario.
Your build up did not sound like it was that bad. No mention of thick gray (indicating lead) deposits in the galley. You can flush that area with your preferred internal (not suggesting engine degreaser) engine cleaning solution, and let it drain to the pan. That's where it goes from there. I would drain the oil before doing that, and let it all run out as you go.
If we go with the idea that detergent oil should not be used in an older engine, then there are thousands of engines out there (many right here on the forum) that are facing impending doom from the change over. It has been theorized here before that considering the fact that detergent oil has been around for quite some time a lot of owners actually started using detergent oil years, and years ago. So unless you know for a fact that detergent oil never started being used then there is a real possibility that it was. Maybe the light build up that you had shows it was not used. Perhaps there will be some comments on that.
If your engine has a filter, or if you add one, (and personally I would add one) anything loosened up by detergent will soon be filtered out. I agree that it is a fact that the bypass filter does not get the oil before it hits the working parts of the engine, but it does not take long before all the oil makes it through the filter. Some do not run a filter, and just do frequent oil changes. In the case of this engine I would use a filter and get things cleaned out.
I may be off base, but considering that you have cleaned the pan, and the valve galley, I don't think that there is anything in there, that serious, awaiting re-circulation, that out weighs the good achieved by switching to detergent oil to clean it up, and keep it clean. Even without adding a filter it will start to clean up as the dirty goes out with the oil change rather than settling back in.