The flagpole vertical sounds like the best idea. But you will need a lot of ground radials to make it work.
Rather than bury them, cut the grass as short as you can. Lay down as many copper wire radials as you have room for, each one as long as possible, preferably all the way to the property line. Clip them in place with metal stakes, made from steel wire, bent like a hairpin. The grass will grow over them and you can mow, with the mower set to cut the grass as high as possible. The clips will keep the mower from sucking up the wire into the mower. After a few months and a few mowings, the thatch from the cut grass will begin to turn to sod, and within a season or two, the radial wires will be self-buried in the sod, and probably difficult to pull up even if you tried. Insulated wire will be better protected from corrosion, but in most soils bare wire will last for many years. Avoid steel or aluminium radial wires, although plastic-covered wire might be OK.
I have a friend in a suburb of Chicago who lives on a postage-stamp size city lot. He has an inverted-L wire antenna for 160m, and about 100 ground radials, varying in length from a few feet to over 100' as space permits. He puts out a very competitive signal on 160m.
But the vertical often makes a very poor receiving antenna because it picks up a lot of local electrical noise; at most locations a separate receiving antenna is a must. My friend uses a couple of small rotatable receiving loops which null out his noise problem very effectively. I live out in the country where I have a full size 127' tall quarter-wave vertical for 160m with 120 quarter-wave radials for 160. My home-built indoor loop often picks up signals I can't even hear with the vertical.