I assume you're supporting this configuration using a simple Windows bridge in Network Connections. Problem is, that creates a single, logical network that's hard to control without affecting your own network limits. And while they do make apps for throttling network bandwidth, they typically do so based on a "per process" basis (e.g. NetBalancer, NetLimiter).
One solution might be to employ a software router, say WinGate. The free version supports up to three (3) concurrent users (not just three seats). Unfortunately, those users are now double NAT’d behind two firewalls, and so it complicates remote access. But perhaps it’s not an issue in your case. Frankly, even licensing is not really an issue since the only client is the hardware router on LAN2. The only reason you’re using WinGate is to bridge those two network connections and still be able to throttle that process w/ the free versions of NetBalancer or NetLimiter.
NOTE: I had considered throttling ICS, but that might prove difficult since the ICS process appears integrated into the Windows firewall service. IOW, it doesn’t run as a wholly independent app/service. And unlike WinGate, ICS doesn’t have any such throttling capabilities itself.
Of course, I suppose you could eliminate the hardware router on LAN2 completely in favor of WinGate if you can live w/ the three concurrent user limit. You might even want to consider WinGate Pro (although probably cost prohibitive for small needs). This eliminates the need for NetBalancer/NetLimiter so that rather than a flat, hard limit, you could throttle based on protocol, time of day, MAC address, etc.