I5 8400 : 6/6, 4.0GHz, 9Mb cache, ebay $50-$100.
I7 8700 : 6/12, 4.6GHz, 12Mb cache,ebay $150+
I7 8700k : 6/12, 4.7GHz, 12Mb cache ,ebay $250+
I7 8700 is the best deal, for right now gratification. Sell the i5 to recoup.
For longer term gratification, scrap the entire platform, go with a moderate lga1200 Z490 and a 10600k or a B550/X570 and Ryzen 3700x or better.
Like said, you've got a decent grade build now. Any small upgrades / piecemeal upgrades will be better performance overall, but nothing drastic and no longer term lasting affects, and leave other parts of the build lagging behind.
It's one of the things I really dislike about the Intel i5 series, any of them since 3rd gen. They are decent for what they are, better than an i3, not quite as good as an i7, but not really worth the expense to move to i7 once they have some age.
Even the i5 10600k is a little iffy, it breaks the mould with having HT, so ends up realistically as an i7-8700k on Steroids.
Kabylake, CoffeeLake, CometLake, all basically revamps of the same Skylake architecture. 11thgen is supposed to be the same lga1200, but different architecture, and bring Intel to pcie 4.0. Hopefully.
Intel might still be king for gaming (by a short-hair) but amd is killing them everywhere else. Don't even bother looking to 9th gen, too expensive and not enough gain.