Not remotely complicated as BIAB, as a side project, me and my friend are developing custom software for 15+ years now (still in process) I would come up with idea and logic, he would code. Personal project.
In any case, there are always more issues arise if several things added at once and much less if we add them one by one with testing in between....

I think the culprit of buggy yearly start is that large number of features are added all at once.
I believe if adding new features spread through the year, with testing in between, there should be less headache and frustrations to address.