Which free firewall OS to use as load Balancer

[Gtx Gaming]

Looking to buy one those cheap mini pcs and slap a few extra network cards in it to make it load balancer.

The tplink one from takealot is super slow and capped 100Mbit, but the cpu can hardly handle a 100Mbit.

So thats why want built one with I5 cpu to handle better speeds.

Currently it would need handle 200Mbit to 300Mbit, but if I ever get fibre it would need handle 1000Mbit thoughput also.

So what OS would you recommend I want switching to backup internet when loadshedding kicks in