Not used for anything heavy, heaviest thing to go in it is my pickup, so roughly 8000lbs. Mostly, it's used for my workshop/storage area, I built a fur room out there, stuff like that, so definitely could get away with a little less concrete

You need to determine the extent of the problem before doing anything else. The contamination may be limited to the one area sampled, or it may be widespread. It may also be just a few inches deep or much deeper. Once you know all that you can figure out how to deal with it.

X2, don't start adding material you can possibly contaminate that would add to the quantity if removal were an option.

Lead is one of those things that is fine as long as you don't consume it. Once you consume it you cant get rid of it. I wouldn't worry about the covered paint or the remodels. I would worry about the dust from the remodels and dirt floor of the barn. When dealing with lead in the factory they hang plastic like Dexter and make that area sealed tight. Add special HEPA filters. Suit up wear masks shower etc.

Best thing to do would be to concrete over it but please consult with an Environmental person. You should have a county person or something. I can help find someone if you need help.

If concrete is too expensive you can probably compact a thick layer of that dirty gravel over top of it like you are going to pour concrete. Again just an idea. You should ask someone that knows more than me.

https://www.epa.gov/superfund/lead-superfund-sites

If you have RCA there, (recycled concrete aggregate) that might be an inexpensive option, it's cheap, the aggregate is fines to 3/4" and compacts tight....you can dust over a layer of portland cement and water it in if you really want to seal it up....