Hey! This is not bad at all! This is a very solid first map, with a lot of care put into the environment design. That's hard when you're still learning the software! The outdoors areas are vibrant and well decorated, the cave exit is similarly well built.
...And the secret exit was certainly a bit spooky...
There's a few things I'd like to point out that are simple fixes.
* There's no music! You can add a Music line to your level header. Look here
https://wiki.srb2.org/wiki/List_of_music for valid music lumps in vanilla. You can also add your own which you can place in the pk3.
An example of this in the MAINCFG would be "Music = GFZ1"
* You don't need to upload this as a .zip! The message board allows you to upload the .wad file outright, completely removing the need for packaging your mods in a zip.
* The level goes into Greenflower Act 2, which is a bit silly for a standalone level. You can add a line that says NextLevel = TITLE, which will take you back to the title screen after the level is complete, and mark the save file as completed.
This would be in the MAINCFG as "NextLevel = TITLE"
* There are lots of texture misalignments! In Zone Builder, if you go into Visual Mode, you can press A on any wall to align the texture offsets.
Regardless, the effort you put into the map is considerable, so Welcome to Releases!