After installing BLTouch and Marlin, my nozzle is too high when trying to print
In some ways, this reminds me of the pain I went through recently. I am also fairly new to 3D printing.
After you have completed the bed levelling using G29 you should have a list of mesh coordinates showing in the interface with Octoprint. (I am using Pronterface, so am not all that familiar with Octoprint) You need to save the mesh coordinates to the control board. To do this enter M500 and then M503 to check that the data has saved. If you enter G28 after using G29, but without saving (M500) the coordinates will be discarded.
When you have set your Z offset, you also need to save that too (M500).
Unless you bed is warped, a 7 x 7 grid (mesh) is overkill. If you are printing on glass or better still, a mirror, a 3 x 3 grid should be enough. I have a 5 x 5 due to may warped bed.
Best of luck and have fun.