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I don’t want to talk you into or out of anything so let me just present some things to consider.
Carl is on the right track with suggesting building an attenuator. He mentioed a “T” pad but you could also use a “pi” attenuator. They do essentially the same thing.
The legal limit for FM part 15 is expressed as 250 microvolts per meter measured at three meters. We can do a calculation to convert this to a power output. The result is if you are using a dipole antenna the power at the antenna feedpoint to produce the legal limit is approximately 11 nanowatts.
To attenuate 5 watts down to 11 nanowatts requires 86.5 dB attenuation. ( dB = 10log(5/(11E-9)). This can be done but in practice it is very hard to achieve due to RF leakage. Even with the proper attenuator the leakage from the coaxial cable will create a field strength much over the limit. There will probably be significant leakage from the transmitter enclosure as well.
I designed and built a VHF repeater and recall what was involved to keep the transmitter power from getting into the receiver. This is not a trivial task. I had to use mesh gaskets on the receiver enclosure and feed through caps. for all power and signal lines into the enclosure. I used double shielded RG11 for the lines. The isolation did not approach 85 dB.
Certainly, it can be done but I just wanted to comment on what may be involved. Personally, I would pursue another transmitter which may be better suited for part 15 use.