Seriously consider adding a small 50 or 60 amp sub-panel in the garage.
Like you, I was in the same situation 2 years ago. I decided to use the tandem breakers and then install a separate 240v line to the garage for my welder. Now its 2 years later and I am planning to install a 50 amp sub-panel in the garage. The single existing 20 amp circuit for the lights and the single 240v 30 amp outlet are just adequate. I've added new tools, more lights and outlets over time. The new planned 240v table saw or electrical heater will not work with the current situation.
I should have just put in the small sub-panel in the beginning. It would have saved me some expensive wire and my time. Think ahead and if there is a chance you are adding tools, lighting or heat, seriously consider a small sub-panel.