Any chance of keeping them in the house somewhere?
Shed or garage – nice quiet places for someone to work and get through the bike lock. Most can be defeated within a few minutes with the right tools and if they think there’s something valuable in there they’ll take the right tools. Still though, with care you may be lucky and avoid anyone noticing so you’d just deter the opportunist. Though if they have a go they might then come back with the right tools or have told someone more professional about it.
Depends how much you value the bikes really (and your insurance). If not in the house then spend a lot on securing shed/garage and then looking at ground anchors and serious bike locks, and even then have the possibility of them stripping what they can off and running with those.
Maybe a heavy duty secure and anchored bike box kept within secure shed/garage (secure isn’t just a padlock – as I learnt).