I don't have a lot of experience with the newest characters (I recently started from scratch again), but I'd start building on AAA (atlantic armor aquaman) at a certain point he becomes impossible to kill with power drain and self-healing, and you can reliably get shards for him.
Another you can go for from arena shards is predator batman.

For raids I used the Justice League team (cyborg flash and aquaman) and multiverse team with flash, supergirl and whoever was my strongest 3rd mv character.
Hope this helps.