It’s not exactly quantifiable, but you can track your self-assessed level of skill and comfort across a range of key skills and technologies, for example:
They’re just some examples, off the top of my head, of some very key things that aren’t going away any time soon.
You could maintain a Trello board with six columns, labelled 0-5, with one card for each key competency you want to track over time, and move those between columns as you feel your skill in those areas is changing. Within each card (representing a high-level skill), you can get detailed and break things down into checklists, and the progress of those checklists may help you quantify the higher level skills.