I’ve had them stolen from my shed as well as from inside my house, several times…
Current set up involves a Viper alarm on the shed door (shed = breeze block construction with oak door). Viper alarms can be set to go off if the door is hit repeatedly, ie before the door is actually opened. They arent particularly expensive. Hinge bolts on the door and security lights.
Then a bell box inside, a silly number of dBs, you cant really remain in the shed while is it going off – you start to go dizzy after a few seconds. there’s a sensor in the shed to catch anyone that gets in through the roof rather than the door.
the bikes are chained to ground anchors, the type that are sunk into concrete rather than screwed into the wall or floor. Ideally you want the chains to be fairly tight around the bike and not trailing on the floor, so that it is harder to use bolt cutters.
I lock the bikes through the frames and then pass the chains through the forks as well.
Not much more I can do, none of this is going to resist a big axe and angle grinder combo, but I’m fairly happy and have insurance.
I think the security by obscurity is a better option in some ways, you dont want anyone to know you are keeping bikes. I’ve also thought (seriously sometimes) about ways of hiding the bikes in the shed, behind some sort of screen or false wall, or keeping a decoy bike in there – but it eventually comes down to security vs convenience.
good luck!