SG-3100 seems a little on the low-end side for an ISP.
SG-5100 is probably closer.
If it was me I'd probably be looking more at an HA pair of XG-1537s with 4-port expansion cards.
Load balancing should work fairly well for them. I'd turn on sticky connections but I would still expect the occasional problem.
https://www.netgate.com/docs/pfsense/book/multiwan/load-balancing-and-failover.html#problems-with-load-balancing