A massive +1 on going external.
Per Apple, even some Apple Authorized Service Providers may be reluctant to work on sealed-case iMacs. Those that do have told me that they must charge two hours labor to do anything inside the case due to Apple's "minimalist" case design and sealed access.
In the NW US where I live, tech labor charge run US$65-80 per hour, so the labor charge can be far more than the SSD can cost.
If your current internal HD still works, you leave it in place for extra storage where speed i not an issue.
As you have a 2017 model with modern Thunderbolt 3 ports, that interface offers a very fast external option but is far more expensive. This is a Thunderbolt external.
[Disclaimer: I have no relation with Other World Computing (OWC and macsales.com) other than as a long-time customer.]
What about performance?
- Your current drive, based on other reports here from that iMac model, is moving data at a theoretical max of about 80MB/sec. In real life and being 5-6 years old, it is probably doing between 50 and 70MB/sec
- Opening the computer—with significant associated costs and hazards—and installing an SATA 6G SSD in place of the current HDD will allow moving data at around 500MB/sec.
- The external USB3 SSD option, that can be accomplished at home by the average end user, will move data at about 400MB/sec
Take from one who has been upgrading Macs since about 1997: in actual use, you will not notice the difference between a 400MB/sec drive and one at 500MB/sec. Either will make your computer feel like a much newer model.
The other external USB advantage is that, when you buy your next computer, the external SSD can live on as extra storage for that computer.
