Huge Quality Loss Issue

Hey! I'm fairly new to posting on Tiktok, so sorry if any of this is common knowledge

I get horrendous quality loss after compression. The videos themselves are high-quality with high bitrate, but as soon as I upload them they become blurry and pixelated. I know some quality loss is to be expected, but that seems too much

I have tried high/low bitrate, different file sizes, uploading from PC/phone, tweaking the settings and video formats – nothing works so far. Is this just how TT's compression works?

Usually, I post from my PC, with the high-quality setting turned on and in 1080x1080, 60 fps, 30 to 40 seconds, H264 or HEVC. Any tips? Thanks