I would like to know how private my tree remains if I make it private but searchable
Surely when you've searched Ancestry trees in the past you've come across entries from trees with are private but searchable? The best way to discover what people can figure out about your tree is to see what you can figure out other people's private trees.
Without a searchable tree attached to your results you won't benefit from any of the wonderful things that Ancestry can do - you lose 90% of the functionality.
This is what I advise in the Masterclass:
"The tree I’ve attached to my own DNA results includes only my direct ancestors – this allows me to make it public without any risk of infringing the privacy of my extended family, but at the same time it provides my genetic cousins (ie my DNA matches) with all the information they need."
Making it public won't help you directly, but it will help your cousins to understand how they connect to you, so will prevent them pestering you for access to your main tree (so helping you indirectly). The chances of anyone stealing your public tree will be extremely low because (a) it's so small that they won't be interested, and (b) because it is so small they're highly unlikely to come across it in the first place unless they're a cousin of yours and/or a DNA match.
You don't have to make it public, but you must make it searchable. And don't include anyone who isn't a direct ancestor as that will make life more difficult for your cousins.
The second decision is whether you make your full tree private but searchable. This would be the equivalent of a tree at Genes Reunited which you haven't shared with anyone - so very safe. I would only advise having a non-searchable private tree if (a) it is experimental, (b) is is out of date and superseded by another tree, or (c) some of the connections are extremely sensitive and cannot be deduced from a public source (eg recent illegitimacy or extra-marital liaisons). In such cases create another tree which excludes the sensitive parts.
Although you can only attach one tree to your DNA results there is no real limit to how many other trees you can have.