I want to find a good Altcoin to invest in before the bull starts, also have a non-custodial exchange. This means, that we have complete control over our funds. If there is, please help convey it, because a lots of new altcoins was born every day.
It's great that you understand that new altcoins are born every day and there are literally thousands of them. That's why always avoid investing in altcoins. Because most of them will have no value in the future.
In the future, almost all altcoins will have no value, but by the next bullrun, some of them may well skyrocket in price. That is what OP is asking.
In principle, almost all altcoins from the top-10 / 20 and the next grow in price during a bullrun. Therefore, it is possible to buy altcoins in equal proportions from the upper groups of the coinmarketcap. Either the same top-10/20, or selectively from the top-50/100. This is what is called diversification. Betting only 1 altcoin may be less profitable than this option.
For me, the only altcoin I have invested in ETH. They are the pioneer in smart contract world and no one can beat them. So my portfolio contains only ETH and Bitcoin. No need for a third one to be honest.
Well, it's not even discussed. If you choose among altcoins, ETH must have.