What's a good way to do your woodwork on uneven grass? A large temporary platform? is that even a thing?
Lots of saw horses. Re-arrange them to suit your cuts and projects as needed, put plywood across them if a flat surface is needed. It still won't be good for precision wood crafting (eg cabinetry) but it'll be fine for general building shit.
Not at any real hobbyist level. If you're using hand power tools yeah sure but otherwise no, not really.
Example, the weight of a jointer would sink and topple probably. Not to mention corrosion.
A shed is useful and the best solution. Don't consider a shed a last resort, consider it highly desirable! If you are too OCD to have sawdust in the house, build a shed if you cannot use or do not have a garage. If you want a solid floor you need a room or a shed, end of story.
You can make excellent sawdust collection systems if you choose to work indoors. A large shop vac (put a panty hose leg over the filter for easy cleaning) makes cleanup easy. I would do this in a shed too.
Working outdoors is only practical if you are willing to move all your equipment each time.
You can get a table dead level on grass by using welding stands instead of saw horses. Most non-welders do not know their wonderfulness, but they are highly stable (designed to hold heavy steel pipe/beam/plate) and you can buy folding units if you want an even smaller footprint.
I have four and want more. Use a digital level and you can adjust the screws precisely.
You can buy stands at your local welding supplier too.
If your work environment sucks you won't make anything.