Cursor hitting "border" between differently sized monitors

Pretty sure there's no way to make it seamless from the smaller to the larger screen.

If you right click on desktop, choose Personalize, Display Settings, you can drag the smaller screen higher or lower in relation to the bigger screen to determine exactly where you can cross over from one to the other.

I'm not sure if UltraMon has any settings like that.

Edit: I just installed UltraMon beta version. The beta version had a Display Settings panel that does that same thing. You can change the relative position of the monitors to one another in order to control what areas will be blocked/open for crossover.

DisplayFusion has an option that automatically moves the mouse to the nearest matching area on the next monitor, making it appear to move smoothly between them.