How to force certain applications to use certain gateway?

I have the following setup:

 ______________
|              |  
| Ubuntu 16.04 |NIC1---(WIFI)---ISP1 (slow, unmetered)-----
|              |
|     game     |
|              |
|  cloud sync  |NIC2---(RJ45)---ISP2 (fast, metered)-------
|______________|

1. I want to use WIFI for certain apps such as cloud sync because its unmetered (ISP1).

2. I want the game and some apps to use RJ45 interface because its fast (ISP2).

For sake of simplicity, lets say Firefox is in category 1 (slow, unmetered), and Chrome in category 2 (fast, metered).

How do I go about doing what I want? We can assume that every application in type 1 has a proxy configuration for accessing internet.

One way I thought would be to run a Squid proxy server on the same box and then use proxy for category 1 apps. The proxy would be configured to use only NIC1.

Would that work? (If so some tips on setting up Squid will be helpful.)

What are some other way to do what I want?