One other workaround could be to define all the UoM under the volume category while keeping the base unit as liter and manually select the UoM from the dropdown while doing the respective transaction. 

You can not do that this way. You must be aware of the selling units of measures and the stockkeeping units of measure and also of variants with different prices. It is not possible to mix units of measures of different categories (units and volume) within the product template - product variant model. If you want to achieve that, you may want to check my little tutorial, it is old, but still valid and gives you the idea on how to do it:

Selling wine by the glass and by bottle while keeping proper inventory