555 square wave generation with frequency feedback
In an astable configuration, is there some way to measure output frequency, compare it with some desired value, and use this to change the resistance in the config? (Basically a feedback loop)


Sure, thats relatively easy, but it requires that you have something to compare it to
Ie. to check whether your output signal is actually let's say 100kHz, then you need a signal which you already know IS definitely 100kHz, and if you already have that then why create it in the first place using a 555 circuit?

All absolute measurements come from comparing the thing that you are measuring to a known standard. In the case of for instance multimeters measuring a voltage they will have inside them a voltage reference, which has a well known, well defined voltage across it which you can compare the voltage that you are measuring to.

In the case of a frequency counter it will have a well defined frequency standard inside it which generates a constant well defined frequency which you can compare the frequency that you are measuring to.