If 15' to 20' radials is all that is needed I may be able to pull that off.
How many ground rods do you think it would take to maximize radiation efficiency?
Do NOT let anyone dissuade you about trying 160 just because you cannot have a large antenna or a prefect system.
I have worked Europe on 160 SSB many times with my mobile, and it has a ~8 foot tall antenna with 12 feet of tip height above ground, and only a truck body as a small ground. I have also worked VK6 and Japan on 160 mobile several times.
You would be far better off, as a general rule, with 20 short radials than several ground rods. I would not expect a ground rod, no matter how deeply driven, to have less than ~50 ohms on 160 meters. More likely it would be higher.
Also, as you parallel rods, they do not parallel in a linear resistance reduction. Two rods are NOT half the resistance of one rod, because the rods "elevate" the soil potential around them. That may be difficult to follow, but the largest change is the first "bite" on the soil in an area. There is a diminishing return and doubling the rod number does not ever halve the resistance unless the rods are a very large distance apart.
Still, just do the best you can.