Installing custom software is NOT harmful for FC3000 and you will NOT brick your handheld.
There are multiple custom firmware roms that you can install on your FC3000 Retro gaming handheld, but to understand the difference between them, please read glossary first.
Name Definition cleanFirmware does not include game ROMS. You can use it as a base for your custom games installation completeFirmware may include preinstalled games. jutleyOld developer of custom firmware neotendoDeveloper of custom firmware (recommended to use any custom firmware's based on @neotendo) opendinguxGroup of amazing guys releasing custom firmware based on @neotendo and @jutleys firmware bobAnother guy releasing custom firmware for FC3000 gramerAnd another guy releasing custom firmware rom with preinstalled games polgamerCustom firmware that included preinstalled polish games
Each firmware can be released for specific FC3000 model.
Version Difference FC3000 V1 TFTFC3000 with 8 emulators and TFT display FC3000 V1 IPSFC3000 with 8 emulators and IPS display FC3000 V2 TFTFC3000 with 10 emulators and TFT display FC3000 V2 IPSFC3000 with 10 emulators and IPS display
When the custom firmware is based on neotendo firmware, you can easily use MakeMyFC3000 tool to patch it for different FC3000 model.