Entry via ESTA after visa expiration/Entry during consular processing

my wife currently has an H1B and I have an H4. Both visas are valid until October 2021. She lives in the USA, I live in Germany. The I140 petition for her EB3-Greencard was submitted in May 2020. When my wife has received her greencard, the consular processing should start. So it will take a long time until I will get my Greencard as well.
Now I have 2 questions:
a) We cannot extend the two visas again. If I want to visit my wife again after the visa expires, I have to apply for ESTA before. Up to now I have always said when entering the country that I want to visit my wife. Will there be any problems if I re-enter the country as an ESTA passenger from October 2021 ? I have booked the return flight.
b) Once the consular processing has started for me, can I still enter the country or do I have to wait until I have received my greencard?

In general, ESTA is given with an intent only to visit US for short time.
You can visit her on ESTA as it is a visitor visa. But, the trick is always that you need to prove your ties back home that you will return back to Germany.
So, as long as you can prove that at Port of Entry with CBP officer, it should be fine. If you have a job and permanent residence in Germany and you have return ticket back, you should be fine. Again, this is always subject to CBP officer discretion.