This is correct, you need a “ closed” loop of outside air (by this I mean, the unit does not vent inside air through the condenser coils to the outside) and the two-pipe units are the only way to achieve this.
If you go that route, be aware that these usually collect the evaporator moisture and you either have to manually empty the bucket or pump it to a drain (you can connect it to a laundry machine drainpipe for example). If that isn’t possible and your indoors isn’t sealed well you might be emptying it a lot. Still, there are situations like smaller basement windows for which this is the only practical solution.
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