I agree with Isabel and Mislav.
I made a special effort to visit Hallstatt two years ago, in early summer.
I was driving anyway so drove over from Salzburg (one of my all time favourite places).
The drive was scenic and uneventful except for finding a fabulous supermarket just off the road a few miles before Hallstatt where my wife and I loaded up with picnic supplies which we took to a roadside picnic bench overlooking a beautiful lake and had a great picnic.
We arrived in Hallstatt about 2:30 and there was not a single parking place in the town. The queue for the northernmost large cer park looked about an hour, probably 50 cars snaking through town. The most southerly, a fair walk back into the town, had about 30 cars in the queue.
We gave up and went around the bottom of the lake and parked near a train station. One or two stops on the train to the Hallstatt stop where we dashed down the hill onto the little ferry boat before it was full, and rode across the lake.
The town was absolutely wall to wall packed.
We didn't enjoy it at all. Scenic enough but I know of others, especially in Bavaria and Baden-Württemberg and Hesse which I like more.
We tried to go into one of the churches for a bit of peace and quiet but it was just as bad as the rest of the town. People were climbing on the altar to get selfies.
I don't need to return.