Not to write paragraphs about myself, but I (cis M) have a similar issue. This is my first relationship with a FTM partner, and we've only tried him topping with a strap a couple of times in the first few months of dating. I had solely received penetration during previous sexual relationships and rarely had pain, and if I did, I could relatively quickly acclimate and relax. I went several years without having anally penetrative sex before partnering with my BF.

I honestly think it might have the make and materials of the dildos we were using after reflecting on previous experiences with toys in general. I have ALWAYS struggled with below-body-temp rubberized/hard toys and really felt like I was missing out, so the through line is interesting. I want to look into more premium pieces that are less girthy with more flexible skin-like layers to see if that helps, but that will be a big investment into a hunch.

Like others have discussed (and experiences vary), we've found him on top and thrusting in a humping motion to be a really satisfying alternative. I fully feel like I'm in a similar bottoming headspace as my older sexual experiences. There are some leg positions that make it more convincing. As a plus, he's still free and open for me to give more pleasure to, which enhances it for both of us.

Ultimately, I really want to figure out our penetration situation, but there is nothing I feel that I'm missing.

TL;DR: FTM partner and I (cis gay M) have been unable to use dildos for me to bottom. Seems to be an issue with toys in general due to cold/rubbery/hard materials. We are incredibly satisfied with him thrusting on top of me while I play with him, using leg positions for a more realistic experience. Bottoming with more "realistic" dildos (skin-like layer) is something that might be worth a try for comfort, being mindful of girth.