This isn't an item to cheap out on. Find one with aptX HD support and decent reviews if you have any room in your budget. I use the bluetooth to FM all the time. I have a pair, one in my car and another in my travel kit for rental cars. The key is to find one with aptX HD and other newer hi-fi codecs as there are older chips still being sold with lower specs as BT has been a thing for awhile now. I just did a search are there are ones that are usb powered or wall powered. Another better method might be a real FCC certified FM transmitter that are marketed for Drive Ins and "church parking lots" and use a bluetooth receiver or a usb-dac as the input.

You drank the aptX kool aid and forgot the fidelity loss along the signal path.

FM broadcasts do a high pass at 50 Hz and stop at 15 kHz. The best SNR is only ~50 dB which is already achieved by plain old SBC. There is no need for higher fidelity audio codecs like AAC/aptX/aptX HD/LDAC besides the fact that most smartphones don't support aptX or aptX HD.