Far cheaper than a high quality new pellet grill is an ATC and some gaskets for your WSM 18.5". I almost bought a new Rec Tec earlier this year. I had given up on my WSM 22.5" because even an ATC could not contain its temp swings, and like you, I was tired of chasing temps on long cooks. Instead, I refreshed and renewed my 22.5" wsm with gaskets, new dampers, new door and a lid hinge. Now that all of the leaks are plugged, my ATC can hold the temp constant. It is almost as easy as a pellet grill; only firing it up is more work.
My ATC is a Stoker (no longer selling new units, just parts), but BBQ Guru, Thermoworks, and others sell excellent ATCs. If you are an electronic DIY'er, you can even build a heatermeter yourself. The current generation ATCs let you monitor and control a WSM remotely so overnight cooks no longer need to wait for the weekend.