I rarely endorse flying Southwest, but if you really cannot plan the travel without needing to change a lot of flights, it does seem like the best option to what you are describing

If you do not know well in advance that you are doing to change the weekend, then the fare difference on top of the change fee is going to be brutal (and likely more than you would originally pay on Southwest).

If you know well enough in advance that you are going to change weekends, then you probably would be able to find cheap enough airfares at that point.

If you assume half your $250 UA flights are going to change, then that half become $450 flights (if you are lucky enough to get the same fare class).

If you book a flight for September, but realize you won't make it that weekend, so you push it back to a weekend in November when you might go, you are paying $200 for the privilege of that change, and you still may find yourself not wanting to go that weekend in November, so now you are going to pay another $200 change fee. A $650 flight even without fare differences.

There are easier ways to make Gold (and the Gold benefits are not all that substantial - you still won't get upgrades). For the additional money you are going to spend on UA, you could do a Premium Accelerator to get to gold.

I would certainly prefer flying UA, and that is a fair number of OnePass miles to accumulate, so by far the best strategy is to try to plan the travel better so that you are not changing a bunch of UA flights.

I would assume weekend flights to Boston are generally cheaper than weekday flights, but with tourists wanting to see fall leaves changing, I bet weekend airfares in late September and October start going up. With the change fees and fare differences, you may find yourself with a bunch of flights that end up costing you $800.