Until last May, I made my living on my 2010 iMac, now retired. With these symptoms, you have 2 choices:
a) Time for a new iMac. Why? Besides High Sierra being End of Life, there's the reason you have given.
it’s my primary work computer.
b) Or this...
If this is the original hard drive, it's a wonder it lasted this long. I service over a hundred if these for schools and all have had the drives replaced. Not one was good when I replaced it though not all were showing symptoms. This is a typical SMART graph of drives from that time with problems. Notice that this shows Passed.
Although this is a Seagate, the WDs often used on the 2009–2012 iMac are just as bad.
Hopefully, you have a good backup. If not, boot from an external drive if you have one and run Disk Utility as many times as it takes. If you get the same error 3x in a row, give it up. I've run DU up to 50x to get to the point where I can recover the data from a drive.
You can try resetting the NV RAM.
https://support.apple.com/en-us/HT204063
If this works, get it to a tech or do the work yourself. Install an SSD and a new battery—a CR2032 is fine. Any tech can install the SSD in 15 minutes but the battery is on the back of the motherboard so plan an hour or more if doing it yourself. Techs around here charge $75 to replace the battery—the drive adds a couple minutes to the job.
You'll need the following:
Temp sensor. There's a way to bypass it (short the leads) if installing an SSD but it is necessary if installing a hard drive instead. I recommend using one with the SSD. This works with either.
https://www.amazon.com/OWC-Digital-...nsor+2010&qid=1566474344&s=electronics&sr=1-1
You can get by with this if installing an SSD.
https://www.amazon.com/Eathtek-Repl...jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ==
Sensor with tools if you want to DIY. You'll also need a small Phillips screwdriver for the bracket.
https://www.amazon.com/OWC-Digital-...ools+imac&qid=1566475383&s=electronics&sr=1-5
Drive. I like these. excellent performance for the price. You'll wonder why you waited so long.
https://www.amazon.com/Blue-NAND-500GB-SSD-WDS500G2B0A/dp/B073SBRHH6/ref=sr_1_3?keywords=WD+Blue+ssd&qid=1566474659&s=electronics&sr=1-3&th=1
Bracket is not absolutely necessary but improves the cooling. Double-stick foam tape works if you want to save the $12.
https://www.amazon.com/Fenlink-Internal-Hard-Drive-Converter/dp/B01ELRRKW8/ref=sr_1_7?crid=15YB6G2GB961Z&keywords=3.5+to+2.5+ssd+adapter&qid=1566474737&s=electronics&sprefix=3.5+to+2.5,electronics,200&sr=1-7
Battery. Apple uses a high heat BR2032. The common CR2032 battery is fine and can be obtained from any drug store.
If you do it yourself, get a bottle of White Out. Mark one side of every small connector before you take it apart. It is easy to force them backwards on reassembly, causing problems when you're done. When you've marked exactly how they go back, it's easy.
