Like this idea, but I think they should take it a step further and offer shares of freetrade as a reward - start at 1 per referral evenly split. Pretty much free for freetrade if they set aside a small block of shares for it early on (and a tiny dilution of existing shareholders), and encourages customers to feel invested in their success. Not worth anything in concrete terms till they IPO of course, but could be worth real money at that point.
Incentives like free alpha are not very attractive for those who already have it, so I’d prefer the shares idea, or a small referral fee in cash.