We have been through this with our dachshund, Morgan, and it can be frustrating but not insurmountable. We rescued him when he was 4 yrs old and a year later he ruptured a disc chasing his ball. From everything I've read this is genetic and 5 years of age is typical for this injury appear. We went the surgery route along with physical therapy and he did recover - for a while. Apparently, he developed scar tissue that put pressure on the spinal cord, so he gradually lost most of the use of his hind legs. He can still occasionally walk but it's not graceful.
I disagree with your vet about the wheelchair. We have one and Morgan loves it. He started using it when he was 7 I (I think) and he’s 11 now and shows no sign of slowing down. So I’m not sure why using it for 6-7-8 years is a problem. IMHO NOT being able to use one for 6-7-8 years would be a problem. It just lets him be a dog again.
We only use it for outside and it gives him the ability to run and chase his ball and go on walks again. He goes crazy when we pick it up because he knows he’s going outside. They do need supervision when they’re in it in case it gets caught on something. Morgan has even managed to flip it over chasing a rabbit in the yard.
I recommend Eddie's Wheels, they custom build the wheelchair to the dog’s measurements. The place where he was getting his PT recommended them and took his measurement for me. I did my own research as well before I bought and didn’t think any of the other options were nearly as good. The website is eddieswheels dot com.
You can also get what’s referred to as a drag bag. Basically, a bag that your put the dog in with harness that goes around the upper neck/torso and protects the legs from getting scraped up when the drag around. We had one for Morgan, but he really hated it. So inside we just let him drag himself around. Our floors are tile and laminate so he can’t tear himself up.
Good luck with whatever path you choose and let me know if you questions I might be able to answer.