Give it a try using one of the various second generation single bay My Cloud “unbrick” directions and let the subforum know how it goes.
I tried to replace the hard drive on a first generation (P/N # ends with “-00”) single bay My Cloud (OS3 only) and found that with the SSD drive installed the first generation single bay My Cloud would not boot. See the following post where I detail my experience trying to use an SSD drive and some discussion on it.
Edit to add: There may be other past discussions on trying to get an SSD drive to work in a second gen OS5 single bay My Cloud enclosure, see the following search link:
https://community.wd.com/search?q=%23os-5%3Amy-cloud-os5%20SSD