a friend used to store in a garage (before investing in a storage cabinet…its not a fridge!! ). we viewed it as short-term storage rather than long term “cellaring”…that was done at a merchants (temp controlled) warehouse
In a garage, the fluctuations are more extreme than you think over the course of the day - whilst concrete is a heat sink there is an awfully large amount of air in the garage that changes temp much quicker - so besides long term 20 deg fluctuations (over the course of the year) you also have shorter term 5 to 10 deg fluctuations (over the course of a day) … this will age the wine far quicker
To help combat this, we made a small “insulated” cupboard to help - basically enclosed the wine rack with a celotex / plasterboard box - reduced the fluctuations down significantly.