While "behind" my Firewalla (Gold) is Surfshart valualbe, or even relavent?

I've been running a Firewalla Gold for several years not, and it appears to work fine.  We have 5 workstations in our home and rarely have used the external access.

In addition, I've been using Surfshark from my workstation ("behind" the Firewalla Gold) and recently have been having trouble that is traceable to the SurfShark.  What occurs to me now -- given we're "behind" the Gold box, is there any real value in using Surfshark to begin with?

Any insight is greatly appreciated ....