I top up my veggie garden every year with compost, animal manures, and occasionally peat moss.

It will sink down again but that's a good thing. It continually improves the soil beneath.

Gots, Hamilton gives good advice, but avoid peat moss because it is not ecologically sustainable.

If you need a lot of fill and must use topsoil, be sure it is free of weed seeds. Top soil is scraped off of farmers fields and contains every type of seeds that was sown or blown in.

"...avoid peat moss because it is not ecologically sustainable."

Some say yes, some say no. We can substitute "fossil fuel" in the above statement to the same extent.

Pertinent to the OP, topping up with organic matter is a periodic task, as OM is consumed. A permanent solution is to add those elements lacking in your soil triangle to get closer to the ideal loam.

More then likely what your Ohio soil needs is organic matter, compost, leaf mold, but not somethjing called "topsoil" which can be nothing of any value. That your garden is at a lower level then the rest of the yard indicates whatever organic matter was in that soil has been digested by the Soil Food Web.
These simple soil tests might be of some help,
1) Structure. From that soil sample put enough of the rest to make a 4 inch level in a clear 1 quart jar, with a tight fitting lid. Fill that jar with water and replace the lid, tightly. Shake the jar vigorously and then let it stand for 24 hours. Your soil will settle out according to soil particle size and weight. A good loam will have about 1-3/4 inch (about 45%) of sand on the bottom. about 1 inch (about 25%) of silt next, about 1 inch (25%) of clay above that, and about 1/4 inch (about 5%) of organic matter on the top.

2) Drainage. Dig a hole 1 foot square and 1 foot deep and fill that with water. After that water drains away refill the hole with more water and time how long it takes that to drain away. Anything less than 2 hours and your soil drains� too quickly and needs more organic matter to slow that drainage down. Anything over 6 hours and the soil drains too slowly and needs lots of organic matter to speed it up.

3) Tilth. Take a handful of your slightly damp soil and squeeze it tightly. When the pressure is released the soil should hold together in that clump, but when poked with a finger that clump should fall apart.

4) Smell. What does your soil smell like? A pleasant, rich earthy odor? Putrid, offensive, repugnant odor? The more organic matter in your soil the more active the soil bacteria will be and the nicer your soil will smell.

5) Life. How many earthworms per shovel full were there? 5 or more indicates a pretty healthy soil. Fewer than 5, according to the Natural Resources Conservation Service, indicates a soil that is not healthy.