Here's the situation:
Conventional toilet flushes liquids with some TP just fine. Everything works and looks exactly as expected. It flushes effectively with a nice swirl.
But when someone makes magic happen, resulting in solids in the toilet, it requires aggressive plunging more than half the time. After plunging, it works fine until someone makes magic happen again, at which time it usually requires more plunging.
Once it is plunged, the flushing looks completely normal again. There is no "slow flush" indicating an obstruction in the drain line. Sinks on the same drain line work as expected without any clogs.
The toilet and pipes are all about 15 years old and have had consistent, but moderate, use. This problem has only been going on for about a year.
I have done some research, and many sites recommend a "main line cleaner" like Green Gobbler. Given that after plunging, the toilet drains fine, I'm not sure if such a cleaner will actually help. After all, there does not seem to be a large obstruction. Also, with over 15 meters/yards of drain line, it seems questionable if even a gallon of such a substance would make sufficient contact with enough of the pipe for a long enough duration to have a significant effect.
How does I proceed to resolve this issue?