This was explored a bit here: OpenVPN Server Redirect Question
The idea is to have two separate client configurations.
On the server site, set it for "LAN only". Export that configuration to your client. That pushes the route to the LAN, but the client will continue to use its default gateway to reach all other sites.
Now copy that configuration to a new file, and add to it "redirect-gateway def1".
So, when you want to use OpenVPN to reach your LAN, but not to reach the internet (as, for example, you are in a secure location and your download speed is better than the server's upload speed), use the first configuration. When you are in an insecure location, like a public hotspot, use the second configuration so everything goes over the tunnel.
While you are at it, configure the second server doing the same, so you end up with four configuration files. I use the second server as TCP/443 to avoid being blocked in two specific places from which I often connect, but the idea is to be able to get in through one working connection to fix a nonworking connection on the other server.