You're right - we don't know what will happen to the exchange rate between USD and STEEM. It will probably go up AND down over the next several months, just like it has always done. That means to avoid taking any perceived losses, I would have to make perfect trades (both into STEEM and into USD) back and forth, nonstop, to always avoid missing out on potential gains. Like I said above, what if I make the wrong trade, and lose some of the donations? I'm certain people would be critical, and with good reason.
The only logical decision is the one I made at the start - to collect STEEM until it equals 1300 USD, the local price of the pump and installation in Venezuela. Initially, I had hoped it would only take a couple weeks for the project to get noticed, and funded. Turns out it's taking months, meaning the USD value of the STEEM held fluctuates. Nothing we can do about that, except stick to the plan :)