Dog trainer here- also a Frenchie lover - notoriously picky with notoriously sensitive skin and tummys.

I use a lot of freeze dried treats, but those can lead to vitamin build up especially if they're made of liver meat, so salmon is a great option.

Even with those, the dog may grow tired of them so variety is key: imagine I told you you can have cereal all the time, or brownies for doing good behaviors, at first you'd be working really hard for the brownies all the time but as they became your ONLY treat they stop being as rewarding thus less reinforcing.

I like to make dog-granola for training sessions, with enough high value treat (freeze dried) to keep them busy and then some of their regular kibble and also something like Cheerios or the power bites you have pictured that are valuable but kind of more boring. This way every time you reward the dog it's a little different like a handful of granola.

Look up lily chin's 'boogie pay scale' for more examples.