A while ago, while working on a game, I was searching for options for creating tilemaps, but instead I ended up starting to write my own tilemap editor for the X16.
It's written in C++17 using SDL2, SDL2_image and SDL2_ttf.
It's still very much a "Work in Progress" but it has the basic functionality to Create/Import/Modify tiles and build tilemaps that can be saved and loaded into VERA directly.
(That being said there are still random critical bugs to be hunted down.)
Currently only 256 colors are supported with no palette offset and the editor only has the default X16 palette build in. In the example folder there's a script for converting Gimp palettes to a format that the editor can use and can be loaded into VERA.
The example folder also have an example, including a C program to load two tilemap layers and a palette into VERA.
The code can be found here: https://github.com/dacobi/tilemaped
There's no OS specific code, although I've only tested it in Linux. But it should compile on any system with a working C++17 compiler and the required SDL2 libs.
Here are two screenshots of the main editor window and the tile editor window: