Automatic Updates are a tried and true way to eventually ruin a good iPhone. Apple has been wrecking iPhones for over a decade this way.
And yes - if you choose to "install tonight", it will automatically choose the newest version of iOS if there are two versions available, iOS 18.7x & iOS 26.x - it will install iOS 26.x Surprise!).
For those old enough to remember - there was a huge interface design change when the Mac went from System 9 (old style) to the new OS X 10 Aqua look. It was a radical change.
But... for the most part, people liked the Aqua interface, it was just slow to use. Not as snappy as the old OS.
As the years went by, the Macs got faster and faster until the OS was fast and fluid.
I think thats Apple's plan with iOS 26.
First version is clunky but as processors get more powerful over the years it will become snappy again.
Nothing drives new computer/iPhone sales like slowing everything down with a clunky new OS.
Thats the plan