Is the top spout clogged with gunk? if you dont know then just unscrew the arms and see if theres food and crap in it and if not check the pores of the arms and see if thats clogged with food just had a similar problem on my dishwasher.
If you can take out the ssd and buy a sata to usb adapter and clear it on another iMac if you have one or have a friend or relative with one. Then boot into recovery mode and put a new os on your ssd and try again the OS may be corrupted. P.S you should probably back up your stuff on your ssd. Sorry I keep refereeing to it as an ssd if its a fusion drive or hdd.
if you usually use your mac book on your lap instead of on stand ups or on a stand you probably got the inside filled with dust if you can grab a can of compressed air. Then try and clean out the side of the computer as best as you can by shooting air through all the vent holes.
You should definitely put it into your iMac instead of a USB 3.0 external ssd. You won’t even need a cage adapter for your ssd just disassemble iMac (take off screen) then unplug SATA cables and put new ssd in.