Yes. Its not as bad as you think. If you find a piece of wood that has a lot of imperfections and holes in it, you can end up with some really cool ones. I have made quite a few. Live edge walnut ones always turn out nice too!
What is used is called “mirror mastic”. (I believe I am spelling that correctly. It’s specifically designed to glue glass / mirrors. I have one hung in my house that has been there for about 5 years with no issues. That being said. YES wood does flex if it’s not settled. Make sure the moisture content is not considerably off from the end point you will be hanging the end product. Our wood has sat for a few years and I still test it with a moisture meter. Good luck 🍀
If you had the materials and the mirror, it could be done in an afternoon. I did it over several days as I would just work a bit and then do other things. Thanks for your interest :-)
