The formation etymology (whether from tin or onomatopoeia) is uncertain. The part that is certain is the semantic chronology. The noun tinker was used from at least the 13th century for an itinerant mender of pots, the Travellers. By the 16th century it became a slur for Travellers.
The verb to tinker doesn’t appear until the mid-17th century, first meaning to work as a tinker and only later coming to mean what you're familiar with.
So while the root word’s sound-shape is debated, the order of senses is clear: the Traveller sense comes first, the modern “casual repair” sense comes later and was derived from it. This is the etymological order given in all sources, eg https://www.merriam-webster.com/dictionary/tinker
The verb to tinker doesn’t appear until the mid-17th century, first meaning to work as a tinker and only later coming to mean what you're familiar with.
So while the root word’s sound-shape is debated, the order of senses is clear: the Traveller sense comes first, the modern “casual repair” sense comes later and was derived from it. This is the etymological order given in all sources, eg https://www.merriam-webster.com/dictionary/tinker