@Yi C.
Regarding #1, all my projects are ground mount. Yes, zoning is always an issue. Many counties have a solar ordinance that may make a project less attractive or potentially prohibitively expensive to implement.
Regarding #2, I've obtained SUPs (Special Use Permits) in two NC counties. The other developers I've seen in the quasi-judicial SUP hearings all used attorneys, even for 500kW projects. I opted to represent myself (as an independent landowner). Yes, these hearings are a pain. And, yes, you do get yelled at and may be required to bring in professionals to debunk the ridiculous claims some neighbors will make. I've had to hire Real Estate brokers and Professional Engineers (PEs) as "expert witnesses" for defending various false claims.
Regarding #3, the smaller your project, the less likely you'll see high impact study costs.
Common expenses would be utility mandated SCADA upgrades for system isolation, metering packages, engineering & testing, etc. I have not yet done a solar+ESS project yet. That is certainly where the world is headed. There are stable chemistries out there, and I'm not concerned much about ESS thermal runaway. I'd consider something like the CATL 1G Sodium Ion, 2-MWh, depending on the PPA.