It is common for SSB to show low values on a wattmeter. Try providing a constant tone, or a long Heeeeeelloooooooow. In another thread, there is a website that has an audio tone generator. You can use that with a 1kHz and the mic close to the speakers. Try different volumes and distances to see if the power out changes. If so, then most likely you have a low audio drive from the mic.
I solved the low SSP audio by getting an amplified mic and using that. Others have gone to a preamp in the rig enclosure, and for the high end, a speech compression amp for maximum punch. I have not that, so you will need to search the groups.io site or the internet for speech compression uBITX.
Hope this helps a little