4 Dogs for 3 people isn’t too much! I have had 3 for myself for the last year (just 2 now, my sister returned from overseas and her dog is now back with her) so that’s just fine
I don’t recommend most senior foods, they are fairly low in protein. I would find an all life stages that works for all of them and see what you can do from there. Switching brands/protein types may help you keep them eating. Large breed adult food, to my knowledge though I have no giants, is unnecessary, and he should be able to be on the same thing as the rest.
Maybe try some of the fromms? The 4 star have a lot of variation and so its very easy to switch what they’re eating from bag to bag. There is grain free and grain inclusive as well
