Take the front wheel off and turn the handlebars sideways to reduce the envelope size of the bike.

Or get a pulley system and hang the bike sideways up against the ceiling in the corner of one room.

Or lock it to the garage somehow.

No locking system is 100% secure against a dedicated or motivated thief or druggie, do your best and get a good nights sleep. At some point it isn't worth the stress of worrying about it.
Good luck.