Export AVCHD 4:2:0 to MP4 with less artifacts?

[AplAy]

Hello there! I was advised from another forum to ask this question here. So here we go:

I have a Legria HF G25 PAL camcorder (which seems very crappy to me or I am just too noob to handle it good enough yet) and I record in highest quality (1080p, 24Mbit/s, 25FPS) and I export to MP4 through a preset called "YouTube 1080p 25fps" in Adobe Premiere Pro CC 2014.

When I upload it to YouTube, I get lots of blurry and strange artifacts (grey pixelated blocks and such) and not as a sharp image (may also be due to poor focusing) as I would like (see example below of I video I shoot; I used manual exposure so I understand the sky got noisy when light outside was lower. There is also dirt on the window I recorded through to capture a timelapse of a solar eclipse):

https://www.youtube.com/watch?v=ZUyO99cduB8

I have asked this on another forum and someone mentioned GOP when exporting but I have no clue what it does and how to experiment with that kind of parameter? I was wondering and hoping that I could get some advice (or a link to a guide regarding this) on how to export my AVCHD 4:2:0 video to MP4 with fewer artifacts (if it's even possible). What I really want to be able to is to be able to upload it to YouTube with fewer artifacts.

I'm looking forward for your replies! / AplAy.

P.S. Here's info about the exported video file that I uplodaded to YouTube:

Format : MPEG-4
Format profile : Base Media / Version 2
Codec ID : mp42
File size : 29.5 MiB
Duration : 30s 0ms
Overall bit rate mode : Variable
Overall bit rate : 8 248 Kbps
Encoded date : UTC 2015-03-20 13:48:43
Tagged date : UTC 2015-03-20 13:48:43
©TIM : 00:00:00:00
©TSC : 25
©TSZ : 1
Video
ID : 1
Format : AVC
Format/Info : Advanced Video Codec
Format profile : High@L4.0
Format settings, CABAC : Yes
Format settings, ReFrames : 3 frames
Codec ID : avc1
Codec ID/Info : Advanced Video Coding
Duration : 30s 0ms
Bit rate mode : Variable
Bit rate : 8 000 Kbps
Width : 1 920 pixels
Height : 1 080 pixels
Display aspect ratio : 16:9
Frame rate mode : Constant
Frame rate : 25.000 fps
Standard : PAL
Color space : YUV
Chroma subsampling : 4:2:0
Bit depth : 8 bits
Scan type : Progressive
Bits/(Pixel*Frame) : 0.154
Stream size : 28.3 MiB (96%)
Language : English
Encoded date : UTC 2015-03-20 13:48:43
Tagged date : UTC 2015-03-20 13:48:43
Color primaries : BT.709
Transfer characteristics : BT.709
Matrix coefficients : BT.709

[smrpix]

You can try raising the bitrate of your export, say to 17mbs average, but YouTube is always going to reencode your upload anyway, so there's only so much you can control.