Is WD Red SSD compatible with Synology NAS?

DS720+ with SSD

I have a DS720+ with Seagate Ironwolf HDD. My problem is that the HDD is very noisy. I was thinking of replacing the HDD with an SSD. (Specifically a WD Red 4TB SSD.) According to Synology this SSD is not compatible with this NAS, but compatible with DS718+. Question is that the WD Red 4TB SSD would not really work with the DS720+ +?

Yes, WD Red SSD is compatible. It is simply not tested on ds720+ therefore it is not on the list.
In fact, every SSD would work on every Synology model (only enterprise XS 2021 models are limited to Synology only drives). SSD are much lower power-consuming devices compared to HDD. Only high power startup HDD might sometimes not work. On very rare occasions.
I hope this helps.

Here is a list of WD 2.5 inch SSD:

Product Numbers