I'm using Tiled. ( https://thorbjorn.itch.io/tiled ) Thank you so much for being willing to help! It's a rare combination finding an artist whose work is so great and who's also so responsive and helps so much!
(FYI the other packs of yours that I'm planning/hoping to use are Candyland, Cathedral, & Foliage Pack for sure, and if we can fix this fairly simply then probably also Castle & Tuscan Seaside, maybe others. Thanks again!)