I also have a Mecool KIII Pro device with CoreELEC (installed on a 120 GB SSD hard drive [Amazon Kingston SSD 500/450 MB/s + USB 3.0 case ~ €32] connected to a USB port) for two months and I have not had any booting problems . My CoreELEC flies at full speed even with a 90-minute permanent timeshift on the SSD for three users connected to the tvheadend TV server and the speed of writing to disk exceeds 80 MB/s without any problem. I have never had to reinstall CoreELEC.
Maybe your microSD has a problem. When you get to start CoreELEC do the following:
1.- Backup CoreELEC in an external USB memory.
2.- Format and install CoreELEC again in your microSD memory.
3.- Recover the backup from your external USB memory.
My opinion about the use of microSD memories as CoreELEC base is not good.