Your log link comes back as a 404 now, so it's hard to see your exact settings. If you can, go to Help > Log Files > Upload Current Log File right after a short test recording and post the new link.
In the meantime, the usual causes of grainy Minecraft recordings:
1. Bitrate-based rate control for recording. CBR or VBR at a typical streaming bitrate is too low for local recording. Minecraft's grass, leaves and distant blocks are fine detail that eats bitrate fast. Switch Output to Advanced, and under Recording set Rate Control to CQP (or CRF for x264). Use a CQ level around 18 to 22. Lower is sharper and makes bigger files.
2. Output rescaling. If your output resolution is lower than your canvas, or "Rescale Output" is on, the downscale softens detail. Record at your native resolution if your drive can handle it.
3. Is it grainy locally or only after upload? Play the raw file from your drive. If it looks fine there and only goes grainy on YouTube, that's YouTube's compression, not OBS. Uploading at 1440p (even if you recorded at 1080p) gets you a higher-quality encode on YouTube.
4. Low brightness. Dark or washed-out footage is usually a colour range mismatch. Under Settings > Advanced, check that Color Range is set to Limited, not Full. If you have HDR on in Windows, try turning it off for a test recording.
Post the new log and a screenshot of your Output > Recording settings and we can narrow it down.