When it comes to diversiying your crypto portfolio there are some things I like to have in the mind. First of all I like to have crypto that is mainstream and not a 'pump and dump'.
We all have crypto portfolios with coins that serves no purpose other then they were free or marketed well but we still hodl them with hopes they make extreme gains and fast money will reward us. For me its the big 3 of bitcoin, ethereum and bnb for new investors.
After all these years, witnessing many cycles, I think it's always best to let BTC be a very big part of your portfolio. Nowadays, I personally stick to atleast 90% Bitcoin. The other 10% can be used for solid altcoins, such as Ethereum as you mentioned. The very last 1-2% (of the 10%) can be used for some non-mainstream / new / quite unknown altcoins or coins that have not proven themselves yet. I only buy them with what I call "play money". It's a better investment then spending it in a casino.