Locking the garage door is the only security I've needed in 13 years.
When I was a student in Nottingham we got some 40mm rebar bent up into rectangular hoops, set them in a couple of 100kg of postcrete, cut holes in the shed floor and put the shed on top. Our bikes were locked to the rebar hoops with Kryptonite New York locks and chains and were not defeated despite a few break in attempts over 3 years.Admittedly this was in the era of boltcroppers before thieves got tooled up with battery powered angle grinders.
Seems pretty solid, but I’m pretty confident it wouldn’t stop anyone other than a casual opportunist. Someone who has spotted your bike, decided to nick it and comes tooled up when everybody is out will get it whatever you do.
With that in mind, have a check on what your insurance policy says, and buy what will make sure they’ll pay out if your bikes do get had.
You could get a bank of these, pir activated or similar, inside the garage. They'd soon scarper. . Might wake the neighbours up mind you.
If the position works with the joist running parallel to the fixing bar - are they generally strong enough to hold? I'd assume so but who knows...
Is it difficult to locate the joists?