It's not so difficult. But depending on your exact model, this adapter (or another one from this website) will work. Carefully check model number before ordering.
https://www.iflash.xyz/store/iflash-solo/
Opening the thinnest IPODs is a bit of a pain. The thicker ones are somewhat easier. Last one (a thick one) I used a thin (and sharp) paring knife to get behind the metal back on the right hand side. Get it started and twist a bit and the back will start to pop off. Slide up and get one side loose. Once one long side is loose, the rest is easy to remove. Be careful and take your time with first one... After 3. I feel like an expert. (But am not).
Once open, replace battery (from ebay for about $5-6.). Remove HDD. Insert the Iflash Solo with a 128 or 200GB MicroSD In and SD adapter. Insert the Iflash Package into the iPod, connect the wire. Place a piece of foam over the mocrosd, sd adapter and iFlash . (Do this to help ensure nothing comes loose and to fill in thickness,). Close the two halves and reattach the battery, but don't snap back into place yet. Connect to Computer and to itunes before you close the case. It should prompt you to restore. After restore move some music to player and test, If OK, then remove cable, and press/snap back into place. (IF you distorted the back metal opening it, you can bend to back in shape easier before you snap it cack into place.) If you lived close, I'd do it for parts cost. Note the very early Ipod classics do NOT use the iFlash SOLO as their HDD;s have different connections. Also note that you can use SD cards, but microsd seem cheaper and work just as well. I have used up to 200GB cards., There ARE adapters that will take 4 microsd cards... But I don't think the ipods wer designed to manage so many music files. SOME models will not accept more than 128GD is you put more in, you'll only get 128GB on those models. ALL this info is in the iFlash website. IF you damage back, opening it, you can buy new metal backs on Ebay (and/or convert to the thin model, as long as you are not trying to use extended/larger battery that the thicker cases allow.)