I can strongly recommend the sureflap cat flap.
Once I’d got Ollie chipped and he was ready to go out, I installed the flap and just had it set up as a normal flap for the first few days whilst he got used to it.
Then I turned on the programming and let him go through, so it read his microchip.
Once set up to use his chip ID, he initially, because of the click of the magnet, was reluctant to come in/go out, so an hour or so holding treats the other side of the flap encouraged him to go through. Now he’s used to it and happy to run in and out whenever he feels like it.
I’ve seen him in next doors house, who have a normal flap, and I’ve seen their cat stare longingly through my cat flap trying to get in.
There’s a little mirror thing on the inside of the tunnel that needs to be kept clean, so if your cat is a dirty little scrote like mine, occasionally you’ll need to wipe it down with a damp cloth. I’ve had to do that once in 7 months, so it’s not really a chore.