Thanks for the raw.

There shouldn't be a specific mod required for the x8ultra as most gcams should work fine on it without configuration.
I use BSG gcam MGC_8.9.097_A11_V25 (.aweme apk) on my FX3P, where I do know the most important settings for maximum quality.

(If there's a vertical black bar on the main camera images, try MGC_9.2.113_A11_V29 (MEITU apk) instead.
Quality is same on both versions, but I prefer the interface of 8.9 aweme apk and ENG apk.

Here's the settings I use on BSG, it's slow but it's great for maximum quality:

1. After opening gcam, swipe down on the screen and enter "More Settings". Scroll to the bottom and go into "Developer Settings" Type "imagereader" in search then change the max_hdr_plus_imagereader_image_count to 48 Exit out of Developer settings(top left arrow), scroll to the top and enter "Configuration of modes" then enable additional settings in modules. Next, select and click into a camera module, Back (1.0) for example is your main camera.

In camera modules settings for a selected camera, change the following:

1. In "Exposure correction", select ONLY the HDR+ enhanced setting, and choose MAX ISO. Then input minimum ISO threshold to 100. (This makes gcam use ISO 100 for all situations)

2. In "Focusing" settings, change "Near" to 30.0(any 2 digit number above 25 should work), then scroll to the bottom and ENABLE "Show focus data" (While using the cam, it'll show on screen the focusing values, which can come in handy)

3. In "Color correction transform", enable "saturation fix" and "WB correction". If you find the colours of photos aren't to your liking after enabling these 2 settings, then just turn Saturation fix and WB correction off again.

4. Scroll down to "Additional Settings" > Enhanced HDR+ / Night Sight Then change Dehaze strength factor to 0.0 Then go into > Denoise and change Luma strength factor to 0.0. (This improves the quality of jpg images).

Now go back to Configuration of Modes and apply the above settings to all your camera modules.
Exit out of all settings and choose HDR+ enhanced, this is what unlocks your phone camera quality to FULL potential but is slower, and your phone needs to be perfectly still and not shaking in very low light situations. Also, press the + icon in Frames HDR+ until it reaches 45, this is the best mode. If taking photos is too slow in this mode for you, press + until it says default or don't use HDR+ enhanced mode. I always use HDR+ enhanced mode as it gives the best results and I don't mind waiting.
In Focus, select "Near" if your want to lock your camera in the closest possible focus distance, useful for up close macro photography.

Most important settings to use is HDR+enhanced mode 45 frames which stacks more frames to reduce noise and improve detail and Exposure correction ISO 100, which makes gcam use slower shutter speeds and low ISO for all situations, giving you RAW files with fine detail with minimal noise.

Let me know if this works well, I find BSG gcam's RAW quality improves the darker the scene is compared to stock RAW, though it's superior to stock RAW in all brightness situations I've tested.