Card counting is possible, but often the casinos will notice and cut the deck shallower(thin for them to win), or wont allow you to adjust your bet sizing.
Counting 6 decks is much different than counting a single deck shoe. It takes much longer to build a "significant" count. It might only take 3 to 5 minutes in a single deck, but it could easily take 45 minutes with 6 decks.
Another thing to consider is when they cut the cards, there are usually way more cards remaining that will go unplayed in a 6 deck shoe. A 1 deck shoe might have a quarter of a deck remaining, but a 6 deck shoe usually has 1 to 1.5 decks remaining.
The best part is if you are physically able to count a 6 deck shoe and the casino doesnt care that you are adjusting your bet sizing and you are successful- the casino will prob hit you with an advantaged player label. Then you either have to make youtube videos about how casinos cheat you, or wear disguises and travel from tribal casino to tribal casino to play BJ.