Personally, I'm choosing to support one or two zoos per month... this just about equates to how much I'd spend visiting them in "non-corona" times. Whilst I don't usually have to pay when visiting zoos (for various reasons), I will always buy food, and something in the gift shop. Last month I donated to Axe Valley Wildlife Park and Lakeland Wildlife Oasis... I've yet to decide what collections I'll support this month, and will probably do so later in the week. Like you, I prefer to donate to collections that I've had plenty of engagement with in the past, or that are active on their social medias, keeping everyone updated about the current situation. That's not saying I'm against donating to private collections such as this, as I will likely donate in the future... but for now, my priorities sit with collections I've somewhat described above.
*Edit - now that the RSCC facebook page has become active, it's definitely a step in the right direction to gaining more support. It's great to see the new developments*