Midea recently recalled its u-shaped unit, so be wary of used ones (I have one and now have no way to get rid of it).
There are also these GE ones that are like upside-down U’s to keep out of the way of the window; but between the unit blowing air so low as to not affect the rising hot air in the room and the fact that they have to pump the condensate up and over the window sill make me think they’re not a great investment…
Call a service tech to look at your central. If it’s not keeping up then it might be cheaper to fix the issues with it over buying a window unit and paying the electricity to run it.
Central Units are typically much more powerful and therefore need to run less, and if it’s something simple like it need to have the condenser cleaned out or the refrigerant recharged the cost to just get it done is much less than the cost of window units plus electricity plus your time to install and remove them