Prior to the merger pmUA
trialed a product where you prepaid a certain amount and were then able to book unlimited flights for a set fee.
My memory's a bit hazy on the details but once upon a time UA seriously thought about offering an unlimited pass...if you search the archived PM UA forum you might be able to find the details there. Vaguely recall it being only available to DEN and LAX as well as some other fine print (e.g. you had to book x days prior to the flight, [ ... ] inventory needed to be available, set-fee was determined by stage length, etc.
Not sure on your situation but does it make sense to consider mileage for part of the journey? For example, if you are certain you'll always be on the Monday morning flight to SEA you can book the cheapest ticket there is. If the return is unknown, you can aim to use a free-to-change-as-a-1K-award to go back.
If awards do not make sense, then maybe use UA for the certain flights and AS/WN for the unknown ones to take advantage of their more generous change policies?