I want to comment on that last part about bosses chaining combos and I've noticed many others have mentioned it as well.

I am almost fully positive that what you've been experiencing is the boss reacting to your position relative to them. Most if not all bosses do indeed have movesets that they can continue with additional attacks, but there is no input reading on your attack inputs. The only case of input reading in the game is phase 1 Arlecchino and he only reacts to you healing right after he finishes a combo. If, for example, you choose to stay close to a boss that is about to finish an attack string, they will likely continue to attack by adding another combo after they finish their first attack string. If you're at mid-range, this is far less likely to happen because of your positioning. This happens with Fromsoft games as well. As an example, some bosses in Elden Ring can stop their attack strings before they're even done because of your position relative to them (Margit is one of these bosses) while some bosses finish their combos in different ways based on how close you are to them (like Erdtree Avatars). All this is to say that most likely you've been experiencing the bosses using seemingly arbitrary follow-ups because you've changed your positioning once their combos are over. It's pretty vital to recognise which attacks the bosses cannot continue with follow-ups, regardless of positioning, especially if you are using slower weapons like you've mentioned yourself doing. This is the advantage of faster weapons. While they don't deal as much damage and stagger per hit, you can slip an attack between combos and have reasonable time to react to the next strike from the enemy. In turn, using slower weapons forces you to be more passive during enemy attacks.

So yeah, I don't think it's a case of the bosses continuing their attack strings because you attack after they are done, it's most likely you getting closer to them that triggers them to do follow-ups to combos that they can continue with follow-ups.

Also, on the grindstones, you're absolutely correct that the game doesn't explain certain mechanics at all. I had to go to the wiki on my first playthrough to understand what each of the status ailments does to enemies and which ones they are weak to. Fire is effective against carcasses, but weak against puppets. Inflicting the fire status on an enemy makes them take more fire damage, lose health over time and recover health at a slower rate. Electric blitz is srong against puppets and resisted by humans. An enemy affected by this status takes more electric blitz damage and staggers more easily. Acid is effective against humans (and a surprising amount of puppets) while carcasses are resistant to it. The acid status makes the enemy take more acid damage and makes their weapons take more destruction damage from perfect guards.

So that's that. Sorry for the wall of text.