But as the object is quite complex, nothing I've found on google worked that well and/or I didn't know how to apply some of the methods properly. I tried using the bisect tool, but you can't quite make a perfectly straight cut and split the object almost perfectly in half, which is needed in my situation as I am planning on 3D printing this to full scale and I need a perfectly straight face the object can lay on while printing.
If you need precise adjustment - open tool properties (left-bottom at 3D viewport editor) and set cut numerically.
For 3D print you would need to have a mesh as manifold, so check Fill option.
After Bisect operation, when cut edges are selected search for Edge Split to separate a half. With mouse cursor close to an one half click L to select linked (or box select.
Or box select a half and click P > Separate by Selection
Press the edit button next to file at the top left of your screen, go to preferences, select add-ons, look up the auto mirror addon. Enable the auto mirror addon. In your object viewport window, in the sidebar, select edit, at the bottom there should be a dropdown menu for the auto mirror addon. Select your model, decide which axes you wish to cut across. Then press the auto mirror button. Now in edit mode, you will see your mesh has a line cut straight down the center, with a mirror addon automatically applied.