You're over thinking it. All you need to do is to provide a way to control power to the transformer that you already have. I would add a relay that provided power to the transformer you currently. Something like this
Amazon.com : SHELLY 1 One Relay Switch Wireless WiFi Home Automation iOS Android Application 1 Pack : Camera & Photo
Pick a relay with app based remote control and pay attention so it has isolation between the relay switch contacts and the power to the relay itself so it can be used on the secondary side of a transformer if need be (this one does that).
Note I've never used this relay, its just an example. But something like this would let you control power to the transformer and that is all i would need. I would link the relay to Amazon alexa and use my voice or my cell to to turn on or off the lights.
I use a home automation system with relays for all kinds of controls and this is an excellent use case for home automation. Its a slippery slope, once you do the pool lights then maybe it's time for other lights, dimmers and soon you got it all automated? I'm definitely on that slope over the past few years.
Good luck!