Which coins would you advise to combine to have proper diversification?
Not who you're asking, but I'll try to answer anyway.
The closest thing to being diversified in crypto is holding anything plus a stablecoin, because it shouldn't move along with the market. It's ultimately pointless because you may as well hold on to your fiat plus whatever coin you want instead, and achieve the same result.
You can also hold anything plus Bitcoin, but since everything pretty much follows Bitcoin's movements, it won't really count for diversification either -- you'll be protected from altcoin-specific crashes, but not Bitcoin crashes because that's very likely to crash both. This is what most people do, however. Pretty much everyone holds Bitcoin while gambling with the more volatile alts.
For proper diversification, I would advise that you look outside the cryptocurrency market. Crypto can be a tool to help you diversify, but you can't really diversify
within it, if that makes sense.
tldr; if you're concerned about crypto diversification, just hold Bitcoin lol