You’ve got bad quality and level at the same time. That means you have a coverage problem. You should focus on exiting the 1800 to the 900, which will normally have better radio conditions.
What are your values for the different Handover margins and priority, as well as CRO?
Since you have fewer TRX on the 1800 cell, you should limit its coverage area, by increasing the rxlevaccess min of 1800 cells.