Which SSD would help my 2017 Mac?

I'm not totally sure if it's possible to boot from an external drive, so if someone else can chime in, that would be helpful.

As far as actual hardware, any modern SSD should do the job. If this is your Mac, then it should support USB 3.1 gen 2. I quite like the Samsung T5 and T7 drives and use a T7 on my Mac Mini to store audio plugins. Just be sure to format them using APFS, as it's the best for SSDs on modern version of Mac OS. Most external SSDs are formatted to exFAT by default.

If you're feeling brave and want to use an internal drive, you can crack open the iMac and install an SSD yourself in the spot where the mechanical drive is. Mind you, I don't know how to do it myself, and the power supply is exposed, so it's not the easiest thing to do.

macOS can boot from an external drive, which is one of the coolest things about this awesome OS! If you don't want to give yourself an aneurysm trying to take apart an iMac, get the Samsung T5 or T7 or a similar portable USB C SSD. If you want, you could get a relatively small capacity drive to install macOS on, and then use your internal HDD for data storage. Essentially, you need to connect the SSD to a USB C port on your Mac and then install macOS to that drive. It might not be as fast as an internal SSD, but it should be faster than the internal HDD. Just be very careful not to unplug the SSD while macOS is running, because that would probably be very very bad for the system!

Another recommendation here for either the Samsung T5 or T7. I also have a Sandisk SSD which has worked well and has similar performance.

My advice would be to completely avoid anything with a read speed lower than 540 MB/s. If your budget allows, I would recommend at least 1050 MB/s. This should give good performance for most typical use cases and also have some overhead for future-proofing. Anything slower than 540 MB/s is likely to be a false economy that you may live to regret. Anything faster than 1050 MB/s is likely to be an unnecessary luxury that won't deliver any real-world benefits unless you have a very specific and demanding use case.

Any USB SSD should be able to hit those speed minimums as long as it uses USB 3. The t5 will do that. I prefer the T7 since it's double the speed. The SanDisk drives mentioned are good as well. The only reason I would pick them over a T7 is if I needed more then 2 TB of storage.

Alternatively, you can go for a Thunderbolt SSD. It can perform much faster, however they are quite pricey.