I am not going flat but I have not had my reconstruction yet. I had a UMX on 3/6, I can speak to what I purchased that helped and what I won't use again.
What I bought and I used and will still use
a pillow set for post surgery that allowed me to sit in varying angles in bed while still staying upright https://www.amazon.com/dp/B0BKPD2CM8?psc=1&ref=ppx_yo2ov_dt_b_product_details
An over the bed table https://www.amazon.com/dp/B000C4QFTY?psc=1&ref=ppx_yo2ov_dt_b_product_details
a foam drink holder for holding drink and stuff while I was in bed https://www.amazon.com/dp/B096N474P7?psc=1&ref=ppx_yo2ov_dt_b_product_details
adult wash wipes, you aren't allowed to shower for a little while, these are life savers https://www.amazon.com/dp/B08627YFGH?psc=1&ref=ppx_yo2ov_dt_b_product_details
dry shampoo - see above https://www.amazon.com/gp/product/B0858KYNY2/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
long lasting lozenges for dry month - the painkillers give me terrible dry mouth, these helped a lot https://www.amazon.com/dp/B011JD6MNU?psc=1&ref=ppx_yo2ov_dt_b_product_details
What I bought and I only used for recovery and won't use again
drain bag - for holding the drains while I am trying to half ass shower or sink wash hair https://www.amazon.com/dp/B096PSKRRV?psc=1&ref=ppx_yo2ov_dt_b_product_details
mastectomy pillow - was good for the ride home and first couple car rides. But didn't need it after a week. I've read both sides on these pillows, lifesavers or useless. I'd rec you get one just in case and if its useless at least you have a new soft pillow. https://www.amazon.com/dp/B09VY28K8S?psc=1&ref=ppx_yo2ov_dt_b_product_details
Edit to add - things I used that I already owned
tons of zipper front hoodies and button down shirts. I was able to put on pull overs but when managing drains the open front shirts are way better
20 bazillion blankets. Recovery made me very cold, IDK if this happens to anyone else but I had to add two extra blankets to my bed.