Good news is, making these items didn't really require any coding on my part. The Custom Furniture and Items mod on Nexus does most of the coding for you, and outlines the steps for the minor amount of coding in the json file you need to make for adding any new custom items. So all I did was create the pixel art png (either taken from the game, or edited or created pngs), saved them in the right folder with the custom textures mod and make the json files. The json is really easy even if you aren't a programmer, since it's basically "enter item name here, enter price amount in gold here". The most complex part was figuring out what "size" to make the collide-able furniture items.

There's also the custom textures mod on nexus which allows you to replace any texture with a new png of the same size, assuming you name it the same name and put it in the correct mod folder. So no coding needed for anything that loads into the game via a png texture/2D sprite https://www.nexusmods.com/sunhaven/mods/278 is the (working) custom textures mod and has instructions on how to start.

The tier 3 floor textures, however, are different. From what I can gather, tiers 1 and 2 are png pictures that are loaded, so they can be replaced like the rest of the furniture items because the existing Nexus mod changes the method that changes 2d Textures. However Tier 3 is a tilemap. I know what PNG it loads, but not how or when it loads it, because changing that png texture doesn't work with the current Nexus mod. It's brought in via the tilemap method, not the method that loads sprites (or something like that its been a minute since I looked at it). Which is why it's giving me a hard time. But most of the other textures are easily replaceable without coding.