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I had an identical situation when I remodeled my kitchen. Even with the old dishwasher under it, the counter still flexed a little in that spot. Since I was installing quartz counters to replace the laminate counters, I didn't want it to be unsupported because quartz was likely to crack instead of flex.

My solution was a large L-bracket that extended from the wall near the dishwasher/oven seam. I ordered a 22" Universal countertop support bracket from CenterLine Brackets (link below). Quality was top-notch, but it did require me to cut open the wall and install 2x8 blocking between studs to give it something sturdy to mount to. I also had to precisely mount it to be level with the rest of the cabinet framing so the countertop would be level. It gave me just enough clearance on the back of my new dishwasher to put the dishwasher in without hitting the bracket.

https://www.countertopbracket.com/Countertop-Supports-Floating-Inside-Wall-Mount-p/fwm.htm

Here's a photo of it installed https://imgur.com/a/jao7Cmm

They sell brackets for this.. sometimes they just behind where the washer opens with rubber mounts.. can get small ones or ones that are just about the width of the dishwasher… or can go cheap and use some rubber shims tucked out of sight lol

Heavy duty L brackets. Amazon has a decent selection

A wooden wedge is all you need, don't overthink it.

I used this exact solution when I remodeled my kitchen. The hardest part is definitely lining it up with the counter when installing but gives good piece of mind

Heavy duty shelf supports. 2 of them, one at each end should be enough. I believe they are 150lb rated each, the ones from Menards are. Also use lag bolts instead of screws when connecting to wall for added strength.

This isn't for load support. Those brackets are to make sure you don't slide the dishwasher when opening and closing the door or sliding the baskets with heavy pots.

If you have room. A vertical gable. A piece of mill work coloured material that goes from under the counter to the floor. You will only see the 5/8 edge when your done.

You should have enough room behind the flange of the dishwasher to use a piece of 1/2" all thread rod and a couple of wall plates which is what would mount to the floor and top ( may have to alter the plates some to keep them from sticking out. I would also check and see if that style of dishwasher would allow you to push a piece of 1/2" plywood up against it and then screw it to the top and floor.

What's on the right side of the dishwasher? If there is a base cabinet filler face frame spacer panel anywhere on that base cabinet run, just either remove it, or reduce its width with a table saw. Then you can shift the dishwasher to the right, by an inch or two. Then, put an end filler panel right next to the oven, with a 1 or two inch face frame filler piece. This new end panel with face filler will allow you to build a 3/4" thick sub counter, to support your new counter. This is how it should have been done to begin with.

I've built a few dozen kitchens, including my own. I see a lot of Mickey mouse, hair assed ideas posted above here, LoL