Phone unofficial service center

I have an old Xiaomi Redmi Note 9 and after updating the software to the latest version, it became slow as hell. I did everything I could to increase its speed, but it is still slow and often requires reboot (like 4-5x a day – it is that bad). Further research online suggests to downgrade the software. I wanted to try but it is quite troublesome, requiring some methods to unlock the device, downloading bunch of files to my PC, and I risk ending up messing my phone. In countries like India (because all the Youtube videos on how to do it are all in Indian languages), there are unofficial service centers. Is there anywhere in Japan (preferably Tokyo area) where I can find unofficial service center to help downgrade my phone?