Hi & welcome! You can absolutely trim the above-soil parts of your plant, and trim its' roots down to a more manageable size. As you've discovered, if left to its' own devices, some potted plants can become too big to manage. This is how bonsai artists are able to keep oaks & other specimens (that would be 100 feet tall if not manipulated) in cute little decorative pots. If I'd only ever given my potted trees bigger pots (potting-up) instead of trimming the roots & replace old soil periodically (repotting,) mine would also be too big to move around, and fit through doorways. A knife may not be enough to remove significant roots, you may need a saw or axe.
It's true one can't divide a woody entity with a single trunk, but I was unsure from your description if all of your plant arises from a single trunk, or if there are multiple trunks emerging from the soil.
Another option would be to donate your tree to a nursing home, church, business that has space to keep a larger plant (after you are sure some saved cuttings are growing well, of course.)
I would so love to see this giant beauty. Please add a pic to the discussion if you can.