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I have worked more in audio but a little bit with RF, and I would say that you can attenuate all the way down to zero (a so-called “dummy load), or to small fractions of 1-watt.
For you I would start by lowering the power as far as possible with transmitter settings, then go the rest of the way with output attenuation.
Radio transmitters usually output at 50 ohms, and if the output is balanced you’d build an H-pad, if unbalanced a T-pad, made from resistors matched to the power they will absorb. I’d even go better than 5-watts and use 10-watt resistors NON-INDUCTIVE, which aren’t too expensive. The pad would be 50 ohms on both sides and have a very specific attenuation in decibels, which I think are the units used for calculating attenuation pads.
There might also be step-down transformers but their design might be very specialized.
Ultimately you’d need to have measurement equipment to know what you’re ending up with, but IT CAN BE DONE!
I want to see what other people say!