The easiest way is to go the external storage route. You can get a desktop USB3 or Thunderbolt 2 enclosure for SATA drives, like a SATA SSD, or for NVME drives. Those two examples of an enclosure+1TB SSD would be less than $70 combined.
You could use it for supplementary storage or you can even install macOS onto that drive and boot off of it. You just boot up in recovery mode (Command R), erase the external drive, and reinstall macOS to that drive and can Option-key boot off there and you're up and running.
This is much easier than replacing the internal SATA drive inside the iMac. That's an option, but it's not easy and I wouldn't recommend it at all.
I have the same machine. I tried using the USB ports as an external boot drive and yeah, it wasn't much faster. What you need to do is get a Thunderbolt 3 enclosure and a fast NVMe and it will rock.
Here is what I use: https://www.amazon.com/gp/product/B0BB74BQVN/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
And: https://www.amazon.com/gp/product/B0B25LZGGW/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1
Using BlackMagic speed test, I get about 2200 MB/s write and 2100 read. There are faster NVMe sticks, this is just the one I had already on hand. You might squeeze out a bit faster.
Good luck.
Pretty much an M.2 2280 SSD is going to work. The Vostro 3470 SFF supports both SATA and NVME M.2 drives. So the long and the short of it, if it fits, it will work.
Now, just because it fits doesn't mean its the best option. The SSD you linked on Amazon is an SATA drive. SATA is super old and is limited to max 600MB/s of read/write speed. And NVME will be much faster and better. Now NVME SSDs have a metric ton of options such as if they have onboard DRAM or use HMB, if they make use of an SLC Cache or if their flash memory is QLC/TLC/MLC/Etc. Its really easy to get deep into the weeds figuring out which NVME drive is the best for your use case. If you're wanting to get into that I'd suggest taking a look at this NewMaxx post to get started: https://www.reddit.com/r/NewMaxx/comments/dhvrdm/ssd_guides_resources/
If you're looking for just a straight up recommendation I'd say pick up this 500GB Crucial P3: https://www.amazon.com/dp/B0B25LQQPC
It will be miles faster and better then the one you linked and is only about $10 more. If you can splurge and do $60 you can pick up the 1TB version: https://www.amazon.com/dp/B0B25LZGGW, but if you can do $60 why not stretch and get the P3 Plus which is even faster then the P3 for just $66:https://www.amazon.com/dp/B0B25NXWC7
this is one of the drives I have set up. works great. I just format it to EXT4
https://www.amazon.com/Crucial-PCIe-NAND-NVMe-3500MB/dp/B0B25LZGGW/
https://www.amazon.com/SSK-Aluminum-Tool-Free-Enclosure-Portable/dp/B0892BK5L6/
1TB Crucial P3 PCIe 3.0 NVMe: https://www.amazon.com/dp/B0B25LZGGW?psc=1&ref=ppx_yo2ov_dt_b_product_details&th=1
I’m not sure why it says the connectivity for that drive is listed as SATA in the details section. The P3 line is definitely PCIe 3.0.