Hi & welcome - sorry to learn of recent loss.
Re the Lab. I have one (working line castrated 4 year old male).
I personally (although he is weighed occasionally) assess by condition. Labs vary in size & shape significantly but a body condition chart can be viewed if you click here
Some vets are happy to sell foods at a profit to them. I would rather feed Simpsons 80/20 than the two products mentioned (in abscence of a diagnosed condition genuinely requiring a specific diet) .You can read re prescription diet products if you click here.
I can make negligible adjustments easily by increasing exercise &/or reducing food over a short duration. I energetically walked my Lab 3 hours off lead yesterday, swimming running & retrieving for majority of that (many other Labs & other similar size working breeds seen plodding alongside owners & looking relatively bored). You mention a reluctance to trot. I advise decent food, (starting at lower RDA levels) & a progressed increase in interesting exercise. I wouldn't waste money paying for vet approved food or for a qualified vet to tell me my dog is 500g overweight
Carbs & Fat each do different jobs (In very basic terms Carbs give dogs off the mark energy but fat gives endurance energy). An otherwise healthy well exercised dog shouldn't be fat unless you are overfeeding (in abscence of a medical issue). I am assuming your bitch doesnt have any other issues that would make her sensibly hold back a little. Mine is currently on Akela 80/20 fish kibble (Relatively low carbs, highish protein, fat a bit above average) Its an acceptable graIn free food & I am happy enough feeding it for now. Simpsons 80/20 Fish has decent Omega3:6 ratio Click here for Omega 3:6 ratio article
& closely considered by me also. If your other dogs get on with it (& it is genuinely appropriate to exercise output), then no reason why all can't eat this (in abscence of evidence to the contrary). I try to avoid high levels of simple carbs for health reasons (you can check my post history for more info) .
No idea if your Lab is show or working type, (the differences can be significant). My response would be easier seeing your dog or a recent pic. I don't consider 22.5 kg as Lab entry weight for "fat club" & wouldn't worry too much re her size. May check if anyone else at home is feeding her tit bits.
Mindful my post is a bit blunt but genuinely trying to help.