Honestly I attribute all my high FG beers to something going wrong during the mash (high mash temp). I know stuck fermentations can happen but they are relatively rare in my experience.

It's probably not done fermenting. If you are using bucket sometimes as fermentation slows the co2 will find another path out around the lid. Take an SG reading in a week and see what it says then - in any case don't bottle or anything yet!

Make sure you are taking pre boil SG at room temp. If you are taking at mash temps you are looking at up to 10-20 points difference. If i need to bump SG i do so after the boil. you can always boil a small amount of water and corn sugar or DME and add it Assuming you had a recipe double check...

Bru N Water has a Alkalinity conversion calculator which is what I use for estimating carbonate and bicarbonate.

Your kettle is large enough that you can do a full volume Mash. It should look something like this depending on your setup. Total mash water needed 18.2 Grain absorption losses -2.5 Starting boil volume 15.7 Boil off losses -5 Kettle dead space -0.5 Hops absorption losses -0.2 Amount going...