Ensuring two leased lines follow diverse routing generally makes them a lot more expensive.
If 3G/4G can provide the worst case backup then it sounds like you are some way from the need for full leased line redundancy.
If you can get FTTC now, then FTTP on Demand in a year or so while not a leased line specification will avoid cabinet power issues and if all underground in your area fairly weather proof.
Plenty of businesses keep one of several types of connection e.g.
ADSL2+ via different provider
Mobile dongles across the various networks
The advantage with dongles is you can potentially relocate office and you can get fixed IP on mobile SIM's
making it easy enough to firewall access to cloud based resources.