Useful input that has made me look again at the straps on my own boat. They protrude just 80mm from the last of four bolts so have nothing like the lever arm of an anchor shank. I'm pretty sure 8mm 316 will be fine. But the side loading onto the shackle does look like a potential issue, though if you get beam on to the sort of seas that see you having a JSD behind you, it'll be the least of your problems!I might move the focus, where all the attention has been so far, to the plates themselves.
No mention has been made as to what they will be made from and it is not clear to me how far 'aft' beyond the hull and the discussed reinforcement the plates will extend.
But I would not skimp and would not use 316 but a duplex grade. At some point the tension will not be in straight line and all the tension, or most of it will be on one plate. I'd be worried it would bend (if it were 316). The weak point is the last or last two bolt holes.
Stainless anchor shanks, or good anchors, are made from duplex.
I'd also check the fit of the shackles in the proposed 'oval' holes. The edges of the holes need to be bevelled (or rounded - I don't know the term) so that you reduce the opportunity for the shackles to lock up in the slot (it happens with anchors it can happen with a chain plate) - and some times when it happens the shackle fails. If necessary I'd take any old piece of steel of the same thickness as proposed) and I'd cut the desired oval holes and actually try the actual shackle in the mock up of the holes - you will, maybe, be surprised at how easy they will lock and then the shackle is side loaded. Shackles loaded at 45 degrees lose 25% or strength (and 50% loss at 90 degrees) and I agree - I've tested it. The shackle holes might be larger than you show on your drawing (you don't mention size of shackle) and the 'least' bit of steel is at the shackle hole. So I'd work out how big the shackle holes need to be and that will determine the amount of steel for both the width and thickness of the plate.
I would pre-bend the chain plate to the appropriate angle to fit the dimensions of the bridle of the drogue.
The plate fits on the side of the transom, obviously, and I might consider welding an end plate, where the plate would be bent as it 'clears' the transom and weld a further plate at right angles to the top of the chain plate to give it more support - if that makes sense. You need a decent welder to deal with duplex.
I'm going to have a think about better articulation there - perhaps a C-link and a few links of 10mm chain or a steel hoop welding onto the strap or even an eyebolt.