There's a woman in the ABA who's done some research on this, including possible effects of various drugs on the baby. The bottom line is that you don't necessarily have to wean. If you're having regular, ovulatory cycles while breastfeeding, then it is obviously not affecting your fertility.
In regards to prolactin levels, it's important to remember that while prolactin is high in the early months, it returns to lower levels even as you continue breastfeeding, so it will not necessarily have any bearing on your fertility.

i suspect FS' that insist on breastfeeding a following the standard, let's do everything we can philosophpy, regardless of whether it actually makes a difference. The thing is, also, while some FS are adamant that you must wean, others are fine with you continuing to breastfeed - they can't all be right.

there was a thread about this somewhere, but I can't find it now. MR's already linked to the podcast

At the end of the day it's your choice to make - how do they know you're breastfeeding anyway? Only if you tell them. I didn't mention it and continued. I thought I would need to wean and started the process, but then found this info and did not stop (we did an HRT cycle). Unfortunately my supply was so low at this point that it disappeared after a week on progynova. DS continued feeding till 8-9 weeks into the pregnancy (despite the lack of milk). I regret that I precipitated weaning. Fortunately the FET worked. If it had not, then I would have weaned for nothing and I'm sure I would feel even worse about taht - my son was not ready to stop.
If we have to do all this again, I will try to keep my supply up gonig into the FET, so hopefully we can continue (so long as my child wants to obviously). This is particularly important to me if the cycle does not work.

all the best with your FET.