Should I compare INJ to an existing well established similar altcoin (if they exists) to understand its potential?
It's not a bad thing to draw up comparison of the coin you're hodling with another that's similar to it in all ramifications. I do that too and it helps me to gauge where I think the coin I'm hodling should head. And if the one I'm hodling is on the lead to the one I'm comparing it with, I should become cautious and make small sales.
In my specific case for example I bought INJ in December and now the profit is about 600% .
At that level of 600% of ROI, you should cut some profit and allow some quantity to roll. One investment tactics I have learnt (after my BNB experience) is never to sell off everything of any coin/token I'm hodling. Always leave some quantity behind. You never can tell how high price will get to later.