Sorry I'm a year late-just seen this item!
I have a 2008 SC430-exactly the same issues-this vehicle is one of the world's notorious parasitic battery drainers. I bought an AGM battery which was marginally better at holding a charge, but have to resort to having two solar panels on my car port roof feeding the thing-even so, I'm lucky if I get more than 2 weeks of charge after a decent run before having to connect up the Ring charger on the Maintenance mode. Like you I don't use the car regularly-this is the core of our problems.
In the winter I use the Ring on Maintenance mode to keep the battery healthy (I don't normally use the car for about 4 months in the winter).
However, for what it's worth, one winter I just disconnected the battery. I'd read all those doom-laden reports of everything being lost, but, when I reconnected, there were no problems at all-apart from having to set up the (only) 3 radio channels I use; I've never programmed the seat settings anyway, and don't recall any other problems.
Don't waste any money having any analyses done; Lexus will also fob you off; there's a wealth of info. on the USA sites about this; but at the end of the day just accept that this is a quirk of this otherwise magnificent car. And BTW there are plenty of other makes with the same issues.
Good luck!
