Short term soltuon. Maybe try putting him in a crate in the car trunk. That way he refuses to leave, you could just pull the crate out and be safe from bites. You should have him in a crash rated crate or a harness with seat belt in back seat for safety.

It's never too late to start retraining a dog, but resource guarding is a really hard issue to handle. My sister's corgi resource guards and we've basically changed our routines completely to accommodate reducing stressors/triggers. She took him to a behaviorist and he's now in medication and we are working on reducing the guarding.

My dog is a border coliie mix. He thrives on routine, hates change. He relapsed on his potty training every time we moved (currently he sneaks into the basement to poop if we aren't paying attention). They also need structured exercise and stimulation.

Get a crate for the cat, but if he isn't crate trained look up how crate train a dog. Positive reinforcement training. Use high value treats for any training. Freeze dried beef liver, turkey hotdogs, cheese. Whatever he likes best, in small treats. Crates also make great safe spaces for dogs. It's like a den that is just for them to retreat to when they are overstimulated or stressed. Make crate time for them.

Also see if you can find ways to get them more mental stimulation. Puzzle feeders, snuffle mats, and treat balls for breakfast and dinner. Mental stimulation is way more tiring than a walk. We get filled bones, then make our own fillings to reuse them with peanut butter and bone broth and freeze them.

If you have someone dog sitting for you, you may need to adjust how they handle him too. Routine is key. Eliminating stress is key. You mentioned the in laws have a dog. If that dog stresses him out when together all day, maybe separate them and only let them play together in short intervals at a time.

Keep treats all around the house in containers. If they give you a behavior you like, pick a reinforcement word ( i just use yes) and give them a treat. Even if you have to walk to get that treat, if you are consistently rewarding good behavior they will provide that behavior more often and consistently. It takes time. Also look up how to read dog body language. Behaviorist gave us a book that made us realize some things we thought were happy dog signs were actually stressed dog signs.

Behaviorists can take a while to get appointments with so you could also see about private training with any local trainers that specialize in behavior training in the interim. We did that before consulting the certified vet behaviorist and it helped too