Is it possible to have a default static route and a default route learnt by BGP active together?

Not a tested solution, but I'd think that if both ISPs links were part of an SD-WAN, which is implemented as Policy Based Routing in Fortigate, you could 'cheat' on ECMP limitation for routes learned via different protocols. You would need to create SD-WAN rules that would distribute traffic by volume to 2 lines equally.