It takes very long to boot up, what could be wrong?

One of the issues we see with Flash storage is the limited re-write of the storage blocks. This becomes an issue with both smaller drives and/or very full drives.

You really need to leave a good portion of the drive unused to gain the best performance. For smaller drives I recommend 1/3 as free space (42GB here) even still smaller drives as they age will runout of clean blocks which spells the end of the drive.

I would make a full backup of your drive ASAP to an external drive! As this might be the last chance. Next I would recreate a bootable OS installer USB thumb drive with your current macOS as you’ll need it to wipe your internal down and rebuild the OS, then you can recover your Apps and user accounts. I would hold off with the data, only installing what is needed today not all of it.

The other option is just get a new drive! Either getting a larger Blade SSD like what you have now or just getting a SATA SSD as your second drive holding your stuff! Then letting the blade SSD be your bootable drive and holds your apps and nothing more.

• Remplacement du SSD blade dans l'iMac Intel 27" EMC 2639
• Remplacement du disque dur de l'iMac Intel 27" EMC 2639

If you put in the SATA SSD you’ll also need a SATA drive cable as Apple doesn’t install it in the Blade SSD systems. You will need to pull the logic board forward to reach the connections.

@lovecd - by chance do you have a power strip which has an on/off switch? Then if that’s true do you tend to turn it off when not using your system and you recently went away for a good window of time?

What might be the issue is the NVRAM battery has died so the settings are getting lost. With the system running open up the Startup Disk control panel and reset it to your drive. See if that fixes things if it does time for a new battery! The battery is on the backside of the logic board.

I would look for a good UPS or get a good surge protector and leave the power on. There was a few units which offered a vampire draw and when the power spiked the rest of the outlets turn on. That way your not dependent on the battery.