Question about render quality

Hi guys, so i'm very new to Resolve and my question might seem stupid but anyway. I've checked the internet for the answer but i can't find it. Can someone just tell me what to do please.
I have this footage i made with my action cam, it's around 40GB in size, 4k/30fps. The quality looks pretty good i have to say.
Now, all i edited in Resolve is a few cuts, i had to cut a few things out. And i added a few transitions in between. That's about it. My goal is uploading this to youtube in a quality that's as good as the original.
Render settings i chose were: quicktime, H.264, 2160p ultra HD, and i set the quality to automatic best.
I expected the file to be even larger... and after more then 2 hours i got a file size of only 3GB. So naturally the quality of that video was horrible.
I then tried another render which was DnxHD, this time the file size came out at 55GB. The quality was a lot better then the first render but still looked pretty bad. Very far from my original footage.
So my question is how can i keep my original quality? I really only want to cut a few things out, that's it. I'm not doing anything advanced.
Please help.

Does the DNxHD look bad or the resulting footage on YT? If your original footage was highly compressed (which is the rule for action cams) and another massive compression is added (like YT, which is compressing its very soul out of it), the result will look bad.
But if you like your footage as it looks in Resolve, I would expect it to look the same in DNxHD.
Can you post samples?

I found out what the problem was. It was setting quality to automatic best. I then tried setting it to restrict to 45k kb/s and the quality is ok. I read somewhere that for 4k 30fps footage you set it to around 40k...