I too was given an NOUO for 7 Watts into a ground plane on a 8Ft pole from ground. I did operate on 87.7 until a Rap station used Frankin FM Technics to lawfully be a FM station. I had to move to 100.1 which was the original frequency I used in 2016 when I first moved to my location.
Watch posting on certain forums too because there are some folks that can go rouge and turn you in as I suspect because when I switched from 87.7 to 100.1 I stayed off FM from Dec 23 2017-Feb 17 2018 and I got tagged after 3 days of 24/7 use. When I did return there was someone who called and said all I played was _ _ _ t because I refused to play Top 40 or Rap as my station was an Album Rocck station which for those who don't know what that is the format was popular from 1967-1984 till MTV was sued. I do suspect this child like sounding person was the mole.
I now transmit C-Quam AM Stereo on 1640 using part 15 power levels and get out around 1 1/4 miles to 2 miles to a Good car Radio and or portable as AM in my area is not too noisy since this is a rural village that I live in. I still broadcast Album Rock and advertise on my town's Facebook page as I'm 100% legal.
I had an FCC agent come and visit my set up on the AM side on the anniversary month I got my NOUO this year and they walked right up to the antenna it looked to my Wife like they took some readings and didn't take long and they were gone and no letter was given and no knock on the door and the station is still on the air.
I do promote HARD C-Quam AM Stereo and very sensitive Radios that will receive my signal at a good distance. I have listeners who do report the signal so you can get listeners on AM its hard and you will need a good audio processor.
Transmission on AM will definitely get out good even with Part 15 power levels if high end audio processing is used as you suggest. The difference in range can be astounding when comparing simple audio limiting to full blown multi-band processing with phase rotation or positive modulation above 100% to tame asymmetrical peaks in voice and music and filtered clipping on the output to catch all peaks and bring up average to peak ratios. Bandwidth limiting for nighttime conditions probably helps range a tiny bit too just like the big stations do by flipping from 10kHz to 5kHz audio at night but I'm not even sure how well Part 15 stations would get out at night on legal channels excluding 1710kHz.
Sorry to hear about your bust. It does happen and it's quite rare for the FCC to take any action until someone reports the pirate on purpose. It's almost always some radio op that wants to play radio police or a disgruntled listener with a cob stuck up their butt. Occasionally licensed radio station engineers will report a pirate if they are cutting into the stations listener base or interfering with their listening range.
Given that you only ran 7 watts into an antenna at that short of a height it's most definitely someone who had it out for you given there are pirates in large cities running much higher power into antennas on tall buildings that don't seem to get the FCCs attention.
The sad truth is that FM pirating is risky to begin with, and broadcasting continuously is not a good idea as that gives the authorities almost no work in tracking down a signal. At least with AM it's usually harder for them to track as antennas can be simple strung up wire well hidden and their gear may not be equipped to easily track signals on such low frequencies. Shortwave is similar in that respect and with SSB and skip it makes it that much harder to triangulate plus most listeners are hobbyists so there is no gain in it for them to report you.
If you look at the FCC database for NOUOs it's almost always FM pirates which is why I gave up on FM because the risk just wasn't worth it anymore for me unless it's for a special event like out at the park where high power can be setup quickly and back down a day or two later.
The only other option is to setup a disposable FM transmitter at some random location and get a secure audio feed link out to it like the guys in Europe do but that's a whole other level of dedication. Makes sense if running a large operation with some funding.