Bacon is probably the most cold hardy variety in Australia. It is recommended for Melbourne, Victoria in the far south and coastal areas around there.
That doesn't mean it won't grow well in hotter areas, early on it was a commercial variety, now replaced with mainly Hass types.
Seeds of this, or known cold tolerant types in cold locations are probably your best starting place, as there is probably less of a jump to increased cold tolerance.
I am not sure there could be a leap to deciduous Avocado trees ?
In Avocados there may be increased frost protection from changes to leaf thickness, ( tougher ) plant inactivity in cold weather, ( semi dormancy ),
oils in leaves ( antifreeze ) etc. Tree shape may affect the way the plant sheds cold air, canopy density may affect insulation properties.
The foliage probably gives some level of frost protection, at least for thin branches and twigs for minor frosts.
There have been -10'C frosts here which devastated Avocado trees in the past.
Some Citrus relatives are cold hardy deciduous ( Poncirus trifoliata ). Cold hardy Citrus X Poncirus hybrids are not deciduous like Poncirus. Some may be semi deciduous, but seem to be cold resistant due to other traits inherited from Poncirus.
Another option may be Dwarf Avocado types, partly cold resistant, and more easily protected by frost cloths / structures.
There are also other Persea species, some may be more cold tolerant, some may be suitable as rootstocks ?