Hi! I enjoyed going through your art, and it's sort of rare to see traditional artists so it's cool see that community still thrive :D
Here's some general critique for you:
1. Try to keep your coloring consistent! A lot of your pieces that use marker seem to bleed, which makes some of the color a bit splotchy, which is very evident in pieces where the background is one large color. It's a bit more work, but coloring with marker a bit slower and carefully hides the bleeding better. A more expensive option is to buy markers that are specially made for artists, such as copics.
2. When coloring, try coloring in one direction! Coloring in one direction unifies the entire piece more, especially when using colored pencils/markers. There ARE times when coloring in different directions can be used, but are mostly advanced techniques to control a viewer's focus and pushing a focal point. Considering the artistic level of your pieces, it would be best to learn coloring in one direction first, and then experimenting with different directions when you begin to learn about creating focals.
3. The stain glass artstyle relies a lot on careful placing of lines to create focals. For your pieces, you seem to be putting the lines in random areas just to break up the shapes. An example of this is
this piece you made, where the lines are very parallel to each other rather than in a more fractured pattern that stain glass is known for. It's a lot of work to make fractal-ish patterns, but I believe it would help make your art pop out more like other stained glass pieces!
For reference, here are some stained glass artworks that use more fractured patterning:
https://images-na.ssl-images-amazon.com/images/I/91CVTrIo0tL._AC_SL1500_.jpg
https://www.stainedglasswindows.com/wp-content/uploads/2019/07/os-e1561995130952.jpg
https://carvingstudio.org/wp-content/uploads/2015/12/IMG_6219.jpg
As you can see in these, the lines don't "repeat" themselves, but rather break up a large shape into smaller shapes. The last example with the lotus flower is a good example to look at. The leaves are their own shapes, which is very fitting for the stained glass artstyle.