Flatties hit the plastics when you lift you rod, lifting the lure up from the bottom.
Bream hit the plastics whilst they are sinking. Bream want the plastic to look like its naturally floating around / slowly sinking.
Bream like a bit of structure like oyster leases, moored boats, wharfs, mangroves.
For flatties I use heavier jigheads (1/4) with a big hook and a 3" plastics.
For bream I use the lightest practical jighead (1/16 or 1/20) with a small hook and a smaller plastic. I use my 6lb setup for bream.
Both like a plastic with a good wriggly tail action.
Technique for both the same. Cast, let plastic sink to the bottom, lift rod, lower rod, wind up slack, allow to sink, repeat.
For bream I keep my finger on my braid so I can feel the bites/know when it hits the bottom.
Hard to go past the nuclear chicken grub tail plastics.
Good luck.