Not a doctor. But YMMV! Talk to an endocrinologist if you can but they will probably tell you that you need to freeze sperm if you want to be 100% sure to preserve fertility. That's what I did and I kinda regret it because it took freaking ages and delayed my HRT date, and I was only able to afford it due to the cushy union healthcare plan covering it through Progyny. It's still going to cost me freaking $800 a year to keep jizz on ice and I have to save for it throughout the year. It's really important for my partner to have bio kids, otherwise I probably wouldn't have done this. If you are able to, you can pay out of pocket for sperm banking and it will be like $1500 supposedly.
You probably don't need to bank before trying HRT. The fertility effects take a while. Lower doses tend not to affect it. If I got a sperm analysis right now I bet they'd tell me I still have swimmers.
Anecdote: A lady I know went off E in order to have a kid with her partner (who went off T to do so as he is a trans guy) and she was able to conceive after going off for a couple months.