Windows defender with firewall enough for small office

Hi,

Kaspersky expiring this month we have a small office of 45 PCs which is protected by Fortinet firewall (web filter, AV, IPS,E-mail Filtering, DLP, Application control)

For last few months I have been hearing positive feedback on Windows defender and one thing I like it newer slow down the PC but still hesitating to make a decision

Is Windows defender is enough with firewall or Third-party AV is preferred what your view?