Is it a bad idea to have only one TileSet?

if your entire tileset atlas fits within one GPU memory page, which iirc these days if about 4000x4000 pixels (but a power of two duh) then it makes no difference. If you use multiple tilesets, each atlas may get its own memory page of the appropriate size. If you use a larger tileset you may get automatic splitting or an error. In any case, the rest of the tileset that is not the atlas image is all individual objects that will have about the same memory footprint no matter what.

Say you are using 32x32 tiles. This means you have to make 1024 tiles to cover a 1024x1024 atlas. that’s how large GPU memory pages were a decade ago. I believe they are 16 times larger, as I said, btu definitely 4 times larger.
Do you intend to have more than a thousand tiles?

If it’s comfortable, put all your tiles in the same tileset, it’s fine, it will be loaded once at the start of the game process and never need to be loaded again, and even better, it will be uploaded to the GPU once and likely stay there the entire time the graphics context lasts, which is good. But this would be a benefit in 3D games with lots of textured polygons. On 2D it’s a tiny benefit that barely factors in performance.

teal deer: do it, it’s fine and good