My iPhone 12 is slow and buggy

Siriggas wrote: "...the battery is in fact original and I don’t have a battery service notification either..."

Even without a battery service notification, the original battery of an iPhone 12 at 84% health is weak and the likely cause of the poor performance. So have that aged battery replaced and all will be well:

iPhone Battery Service

About genuine iPhone batteries

Repair Provider Verification

(Articles found by searching Apple's Knowledge Base.)

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Siriggas wrote: "I bought a refurbished iPhone 12 about 6 months ago (...) The battery showed 85% health when I bought the device..."

If the iPhone was refurbished by Apple, the battery health should have been 100% when you bought it – because Apple's refurbs have new batteries. Is there an "An Unknown Part" message? — See the Parts and service history section here:

About genuine iPhone batteries - Apple Support

...Found by searching Apple's Knowledge Base.

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84% health will not slow it down. Apple will not replace a battery above 79% health.

Before you go there, did you install VPN? That will make it slow.

Back it up, Completely restore it to factory settings then set it up manually without restoring the backup. See how it runs. You will have to reset to factory before a repair.

If it isn't the battery they will charge over $400 to fix anything else

Your device has probably had its original parts replaced, as it is an iPhone 12, it would be worth going to an authorized Apple service center and asking them to check your device.

You should return or sell it and should buy latest apple device

It was not refurbished, someone gave of a clean up and sold it as refurbished.

Steer clear in future. Buy new or from the Apple refurbished store.

Return a used phone after 6 months?

I love an optimist.