Can’t provide input on very bulky items, but It seems you don’t use panier bags or s front basket. Those helped me do much with most items. Some slightly bulky items as well. Add some old tires tubes (free ones with holes from bicycle shops) and I can hold a lot of things.

An e-bike might be a better option if you have many hills and are carrying a lot of weight, if not a proper e-cargo bike. But for the former you need to be careful about how adaptable it is to racks and fenders etc, since some are not great.

I have a bicycle crate in my rear rack (40L from memory). I can just throw my backpack and/or shopping in there and be on my way. No issues transporting when empty. I avoid riding in the rain but I guess a waterproof bag would help for that. It’s durable, the main concern is the rear rack. I had to replace the cheaper rack that I bought last year after the welding snapped in a few places over time (I had it held together with duct tape for a while). My new rack should be much more sturdy this time around.

I have access to borrow a car which I do every few weeks so I don’t need to over engineer my bike setup too much.

I’ve used (and do use) a lot of different things. One of my bikes has a front basket which is very helpful. I have a pair of large panniers that clip on a rack on each side and a smaller one that folds over the top of the rack. I have a long-tail cargo bike which is amazing, it’s got almost as much space as a USA shopping cart. I have used a trailer, mostly for pulling kids but they work well for cargo and usually clip on and off quickly and some fold up flat for storage. The long-tail cargo bike is the least appealing if you don’t have somewhere at street level to store it, but I think it’s the best option if you do.

I went foot only for a year in Minneapolis, and before that I went bike-only for a year, and if I were in your shoes and had a bike and could use a bike I would get a Burley in a heartbeat. You could haul 2 cat carriers behind you and possibly a third on a rear rack if you were so inclined, and groceries would be no issue. It’s half a car trunk and doesn’t really affect the balance of the bike

When I was young and healthy enough to use a bike, I went shopping with a trailer for my bike. It was fully capable of transporting 60 kilograms of stuff. Was a bit tough at the end of the trip when the road went uphill at the end, though.

Most wagons will suck on stairs. Heavier loads will be difficult to manage on hills, both going up and going down. Curbs are usually manageable.

In general it is usually easier to pull than to push.

I’m not car-free, but I do all my grocery shopping without a car. In fact, I’m at my local grocer as I type at 1.7 miles away taking 250 feet elevation gain and 210 feet loss. I understand your pain!

Here’s why I still think a bike fits your situation. Namely an inexpensive folding bike with a front basket and rack.

Hills are conquered in the same way as a cart: walking uphill. Also known as “hike a bike.” Folding bikes are usually also allowed on busses, so you could take a bus one way. You could time your departure to a bus schedule and shop knowing you don’t have to rush or spend a long time at the store.

Folders can be brought inside and consume about the same space as a folding trailer.

My overall point is a folding bike is a trailer that you can ride downhill in. Electric would be a nice upgrade, but it’s not necessary.

First the unhelpful answer:

I transport groceries with my e-bike and a bike bag on either side of the rack. They may not be quite as fancy as the ones you can unhook and carry into the store and the design is probably from the early 2000s, but they’re BIG, so they’re great (apart from the fact that they create so much drag that cycling without the motor is an ordeal).

Other than that, I did use a shopping trolley for a while. It was fine enough. Wouldn’t recommend to transport cats in those though, you probably need something with tires, not just wheels.