There is a lot of information here, but I think it got all a bit too mixed together making it unclear what might actually case the disk I/O. For a start, upserting sessions in mysql does not mean it would sync to disk all the time, so this may or may not be related. Also it is unclear to me when and why how much disk I/O is expected and when it is an issue. So it becomes even harder to properly respond here.
Maybe we can try to separate the issues mainly first focusing on the potentially unnecessary session creation by the healtheck and that also ideally one application at a time. Maybe you can create those issues at the individual app packages to track those better, otherwise those issues easily get lost until such time we have resources to look into those.