I guess we shouldn't be surprised that surgeons are more worried about doing a good surgery than about how clearly we understand what's coming next. At 2 months post surgery you may still not be at a baseline? You are looking for that nadir (low point) while also measuring very tiny amounts, leading to random fluctuations as well in what you're measuring. It would definitely be nice if it keeps going down.
The goal, of course, is to be neither early nor late, since the treatment goals are not cures, but rather delays in progression. In other words, rather than putting the fire out once and for all, you're looking at the advancing enemy and deciding when to fire your limited supply of ammunition.
Perhaps fortunately, the advancing enemy is moving in months and years rather than days and weeks.
So the first task after surgery is recovering function and enjoying the present. If tracking an elusive PSA makes you happier, why not? I don't think monthly blood tests are particularly deadly, just "medically unnecessary." But any MD should be happy to look at more data, even if they just say, well, that doesn't really change anything. After all, they spend most of the day every day at work looking a data (except, depending on how you define data, maybe doing surgery?!)
BTW, remember that different labs use different assays (recipes for analysis) so results aren't directly comparable although they should be close since they are attempting to measure the same thing, so if you are going to one lab that the MD orders and another in between, you really have two different series of results to compare--the trend lines should be the comparable, but not necessarily the numbers.