However, there's an important discovery he hasn't made. This won't work for everyone, but I'm betting it would've worked for him given his high level of fitness above the waist. When you're working at ground level, it's not necessary to fold yourself in half. Use a flat scooter. These come in various styles. I've seen several of these in use in foreign countries where people can't afford the usual wheelchair. They're not ideal for everyday use but are much better for certain tasks.
Gym scooters are usually square or round, meant for sitting or kneeling just above the ground. These are great for housework or garden work where you need to be low and inch along periodically to a new spot. They work for a variety of mobility issues.
Mechanic roll boards, now called car creepers, come in a wide range of styles. These give you a lot of support for lying flat, so you don't need much core strength and don't have to cross or fold body parts. The ones I was familiar with were basically just a long board with wheels at the corners. Now they come in fancy versions with padding and joints, which makes them a lot more useful. The high-end ones can be bent into multiple shapes, very useful for lying on your back or belly, sitting, kneeling, or whatever you need. The main advantages of the old kind are that they're a lot cheaper and they're equally good for lying on your back or your belly, or sitting, or hauling stuff beside you. Many of the modern, fancier ones tend to be custom-shaped for lying on your back, making them better for that and worse for everything else. So think about whether you need more comfort or more versatility. There are instructions to make your own in welding.
I also found something new to me: creeper seats. These are little stools with a tool rack. The high-end ones have adjustable height. Consider that lowering yourself just a few inches may make the difference between being able to reach the ground or not. Also, the lack of back/arms means you can lean in any direction. (You need good core strength to stay on, though.) These are related to something I already knew about, rolling garden seats. Those have different designs with much bigger wheels. Basically, you want the mechanic version for working on a flat surface like a patio, deck, or indoors; and the garden version for grass, mud, or other rough terrain. There are instructions to make your own in woodworking.
Mounting and dismounting: depending on your mobility impairment, you may need a transfer station with a seat shelf and/or grab bars to get on and off a scooter.
An advantage is because these scooters are meant for abled people, they cost a tiny fraction of medical mobility equipment. Also if it stops working, you can often fix it yourself or get a mechanic friend to do so, rather than lying around for weeks until an official wheelchair repairman can be arsed to come fix your gear. A disadvantage is that insurance probably won't cover it.