Hello, good work so far.
Find is useless for finding many things, get Find Any File...
Hold Option or alt key when selecting Find to Find All.
But also, go to Users folder at root of the drive, do a Get Info on the old user's folder, unlock the lock & change rights so you can Read & Write, Apply to enclosed items.
Well, if the Get Info method above doesn't do it for you, there's a simple but crazy powerful tool...
Might try BatchMod, it's much better/easier than the Finder for recursive Permission changes, but careful, it's powerful...
Once you open BatchMod, drop that whole older user's folder on it's windows, check the R&W boxes & the Apply to enclosed items.