I'm 5 weeks post breast augmentation, under muscle. Can I wear seamless bras that are slightly padded?

Best to wear whatever you find to be comfortable. Just remember that you will not see your final results until 4-6 months so no need to invest in an expensive bra right now. Find what fits well, knowing that you may have to change bras in another month. Good luck!

After
breast augmentation I find no particular advantage to the use of surgical bras.
In fact, I find that they can do more harm than good. They hold the implants
too high, preventing them from dropping into position. In addition, they can
put pressure on the healing breast or the wound. They provide no advantage. If
the surgery was performed correctly, external appliances after surgery are
unnecessary to achieve the appropriate final implant position. At one month we
allow the patients to wear a soft cup bra with no underwire. At two
months, they can wear any bra with a wire without push up. Before three
months it is not wise to buy a whole new set of bras. By six months, the
bra size is not going to change. You need to try bras on. Being
measured is almost never productive and almost always oversizes. Find the
ones that you like the way they fit. If the bra is comfortable it is okay.  If it hurts, it is not helping you.