Since you cannot change the OS, you can still customize the experience using "debloating" techniques:
Avoid "all-in-one" toolkits from random YouTube videos. Many contain malware. Always verify checksums (MD5/SHA256) of your downloaded ROMs. nokia g300 custom rom
The G300 is not a developer-favorite device. As of 2026, there are no stable, daily-driver custom ROMs (like LineageOS, Pixel Experience, etc.) available. You may find scattered, unfinished builds on XDA or Telegram, but they likely have broken hardware (camera, 5G, fingerprint sensor). Since you cannot change the OS, you can