dont chain complex multi-stop trips into a single flight itinerary, itll limit your choices and yield higher prices. thats because the booking engines try to keep the carriers within a partner networks.
book the main transatlantic trip first, so austin to berlin or warsaw, and the connections separately.
in this case the itinerary is not too complex so you could try searching for a “multi-leg” itinerary (aka “open jaw”), austin to warsaw & berlin to austin; book warsaw to berlin separately.
btw there is only 1 carrier serving waw-berlin, its the national polish carrier LOT. there are also tons of direct trains taking only 5 hours. given annoyance of going to/from airport, waiting, baggage cost, etc, train might be much better experience.
warsaw is actually quite unpopular for tourists, but i like it a ton. its super chill and people are nice. hotels selection are good. the marriott, westin, hilton (obama stayed there) are all very reasonably priced. for 4 adults staying in 1 place the Platinum Residence serviced apartment is a great option too. (ive stayed in all of them).
independent hotels in germany are typically a bit…err..weird. holiday inn and holiday inn express across germany are very reliable properties, as are best westerns.
book small, specific tours as you need at each place. in 2024 with smartphones and cheap data access and everyone pretty much speaking english you dont need a full hands-off tour to do all the little stuff for you such as getting around the city