Following the ruling of our site’s halachic supervisor, Rav Yehuda Henkin, either option is halachically acceptable.
If you freeze the sperm in advance, then you can time your collection for when you are not in niddah, leaving you with a few options: having relations with a sterile condom, your providing manual stimulation, or your husband manually stimulating himself (preferably, with his non-dominant hand or a glove). Please see here for more details.
If you choose to collect the semen just prior to the procedure, however, there is a chance you will be in niddah. That would limit your husband’s options for semen collection to stimulating himself.
We invite you to arrange a free consultation with a Yoetzet Halacha Fertility Counselor to provide halachic guidance throughout this journey.
We wish you much hatzlacha!