SciPost Physics would be an alternative: good quality, open access, free to publish. However, you might first want to check if your subfield is adequately represented in their editorial college: https://scipost.org/colleges/physics
Usually appeals are not a good idea, but in this case it might be your best choice. It is very odd for an editor to "suggest" appeal.
Appeals can only be on the basis of procedure (not science!). The procedure says decisions are based on review by referees. Five of six referees said ...