Google flights has a lot of great tools, but it does take some effort to find exactly what you want.
First, here's their explore page where you can put in a single airport and with applying the filter for flights only and nonstop only, I can see that the only commercial airline flying into SJT is American and they only fly to and from Dallas. So any flight to SJT is going to have to stop in Dallas.
https://www.google.com/travel/exploreNext I'd compare some basic flight searches from either BOS or PWM to DFW and then SJT to see the price difference. Not looking at specific dates, but seems to be about a $300-$400 difference. So this part is easy and that you can consider whether the extra leg from DFW to SJT is worth ~$300 or whether the car and time makes more sense at that price point.
Once you've kind of identified the route you want to go, use their regular flight search and put in the first weekend that you'd consider doing.
https://www.google.com/travel/flightsSo for example BOS -> SJT on the May 9th - May 12th. Once you've done the search, and applied any filters that you want, there are two features above the list of departing flights. The first is called "Date grid". This will show you a table with the prices on surrounding days. So really helpful if you have a day of flexibility, like you could come back Sunday or Monday and this table will show you if there is a potential price difference.
But the other feature, which I think is more helpful for you, is the Price graph which shows the prices for that route at the set # of days of the trip for every possible departure day. This will allow you to see whether the 9th, the 23rd, or some other future day might be cheaper for the same 3 day trip.
You could also consider other small airports nearby. Looks like Abilene could be another option.
It's going to take some fiddling around, but hopefully these tools can be helpful.