Welcome to the forum.
I can relate to some of what you've said. I was diagnosed 7 years ago and I've been on Sinemet Plus for most of that time. At first it was fine and it helped a lot with my symptoms (which were like you described: feeling stiff / slow and slight difficulty walking) but after about a year the side effects really kicked in.
You don't say what side effects you've been getting from the Sinemet, but I was having really bad involuntary movements (dyskinesias) which I put up with for several years until about a year ago when I was prescribed a drug called Topiramate which has helped to counteract it.
I'm also having a few problems with speech, although for me that seems to be more of a recent thing. It wasn't one of my early symptoms like it was for you.
I hope your appointment with the Parkinson's specialist goes OK. You'll be able to discuss the Sinemet side effects with him / her and hopefully agree on some treatment which works for you.