Congratulations on your new 835. As you can see there are many satisfied owners here on OG. I've owned one since shortly after they were introduced and currently have a black one with the Matthews Recoil Reduction system and the LPA trigger. It's a fine gun.
There are several good chokes that work well with the 835. The Hevi-13 Turkey Choke is a very good one as is the SSX. They did best in my work for the NWTF with 3" WLB shells with 1.75 oz of #6 shot and also with 3" Hevi-13 shells with 2 oz. of #6 shot.
It's been my experience that the 3" shell will do all that's necessary and with much less recoil that the 3.5" shell. That can allow you to get back on target much more quickly and also help you not develop a flinch from shooting at the patterning board with those longer shells.
Over the years I've found another constant and that's a "deep cleaned" barrel will allow you to see how your gun patterns best
https://allaboutshooting.com/blogs/blog/98772615-deep-cleaning-a-shotgun-barrelYour gun may like to be squeaky clean or a little bit fouled. If you start out clean and don't clean between shots, you'll be able to determine what yours likes.
All my best to you with your new gun and in the spring season.
Thanks,
Clark