As someone who once spent around 15 years installing and maintaining security systems of all types all I can say is if they want it enough, they’ll get it. All you can do is make it harder; the longer it will take them the less appealing it is to the burglar. I’ve seen all sorts over the years and you’d be amazed the lengths some of them will go to if they think they can get away with it. Steel doors cut through with Stihl Saws, walls penetrated with all manner of vehicles… the list goes on.
I’m a few years behind the curve these days but monitored systems were always the most secure. The top of the tree would have been RedCare GSM signalling to an ARC but that costs money and isn’t infallible. Their USP was actively monitored telecom lines but the advent of GSM based speech diallers has got around that and it can be implemented for a couple of hundred quid all in without any ongoing monitoring costs.
Off site cctv recording/storage is deffo worth thinking about. I’ve been to more than one premise where the burglars have simply taken the DVR and/or drive caddy’s (and evidence) with them.
Never underestimate the power of deterrence. Decoy camera housings are cheap and really do work sometimes.