You can definitely go to a 36 tooth cassette in the rear with no issues (your current chain likely will cover that extra small amount. of reach) and that will give you about the same lowest gear as going down 2 teeth on the front. You cannot just go to a smaller inner front ring without likely affecting shifting performance since GRX 400 front derailleurs are designed to shift a 16 tooth difference between large and small ring. You may be able to switch to a smaller 2x MTB crank and derailleur matched to that crank, but I am thinking that is more than you want to do.