Not sure why so many people think you need to dive right into water chemistry. I would say that your symptoms - low attenuation and off flavors - are more indicative of a problem with yeast health, fermentation process and perhaps sanitation. I am not saying you should never learn about water chemistry but that's not your #1 priority. Probably not even top 5 at the moment.
Start with good healthy yeast. Use liquid if you can. Use a starter. Good sanitation and temperature control which you already have. Some patience. Read some literature and keep educating yourself.
+1
Here is a thread with the two leading homebrewtalk water science contributors commenting on the impact of water chemistry on attenuation. One says sure it does, a little. The other says nope.
https://www.homebrewtalk.com/showthread.php?t=493150
I suspect temperature control issues compounded by changing seasons. You didn't have these issues with extract brewing but have with all grain. US-05 is a great yeast that responds really well to active temperature control. I like it pitched at about 70, held at 68 for first week of fermentation and raised 1 degree per day to about 71 as krausen starts to drop.