Is there any software for openwrt which switches the default gateway for failover.
I looked at mwan3 but its too complicated for my use case and requires interfaces.
It should ping through gatway1 and gatway2 and set the default route accordingly.
Yes this can be done with MWAN3. LAN => WAN & WAN2 you aren't changing the route. You are just providing more than one WAN gateway. If that makes any sense at all.
On that interface you can go DHCP options => Advanced
In the DHCP options enter 3,192.168.0.1,192.168.0.2
What that does is tell your clients that get an ip address that there are two default gateways. Of course if you have statics that need this you'll have to manually set that in addition to.