You're definitely underpaid. Growing up, I also had a mild stammer, and went through a couple of weeks speech therapy (not sure what age, sometime around 10-11) after speaking to my school nurse about it. You may want to speak to your local GP and see if you can get some help. A lot of what I learned was just about how to slow down my speech and breathe, which felt a bit silly at the time, but over the years it has proven to be valuable. I'm sure you'd also get some value from some it.

You're also fortunate to be in an industry where online communication is the norm, and it should be fairly easy to pick up side work and build up your skills and experience that way too. During my years freelancing, I worked for clients where 99% of communication was done via email, Basecamp, etc, in which case having a stammer is a non-issue as very little communication is voice.

Good Luck!