Backpack/grooming/harnesses thoughts and suggestions?

Backpack: I would love to get a cat backpack for this chonkster but very worried that he soon won’t fit, or will eventually be too heavy to lug around on my back (esp as I have FMS). Just wondered if anyone has any experience with them or recommends any that would be big enough. He is 8 months old and 6.7kg last time I checked so defo destined to be a big boy.

Greasy boy: Since I got him his chest and belly has gotten greasy. I know the breeder would put baby powder on him which I do not do as I’d imagine that’s extremely bad for him. But I think it’s contributed to his greasiness, like how with humans, our hair only gets greasy as a result of shampooing it so much, and if we left it alone it’d sort it self out. I just wondered if anyone had any suggestions for this. I do try to brush him but he attacks me almost immediately so it’s tough! I have also washed him with proper cat shampoo a few times but the grease comes back within a few days. He hates being washed. I have wondered whether dog groomers would take him but am yet to ask.

Harnesses: He has a harness that suits him so much but he escapes it. I would like to take him out in the garden when the weather gets warmer but ofc don’t want him to wriggle out. Anyone have any suggestions for good Maine coon harnesses?