I don’t have much helpful to offer, but we are on a similar path. I am on my 2nd IUI with Letrozole and have one follicle at 23. I don’t actually know what the follicles looked like during first cycle had because they didn’t ultrasound after day 10 (when there were two follicles around 10, one in each ovary). We did the IUI at day 22, so there was no up-to-date follicle size.
It looks like we will be switching to Clomid as well - although we are talking about trying one more round of Letrozole. And of course, this is assuming that this round fails.
I’m a little nervous about the increased risk of twins, but I would much rather have twins than none! And the risk of higher-order multiples is still really small with Clomid.
Hopefully neither of us will have to deal with the decision to switch to Clomid - and this cycle will be the winner for us.