you have to show clearly that you don't want GC

forfeiting 485 can not prove you don't have immigration intention, since you can't prove whether you willingly chose to forfeit 485 or failed to proceed 485 any more.
The way to clearly prove: after you get green card, mail it back to USCIS with a letter stating that you don't want it any more and ask them to revoke it. Keep that record, and it would the iron-clad proof that you don't have any immigration intention whatsoever.