Split DHCP on Subnet

I've got an unideal situation with a few options. The option that will likely be chosen is to expand the network from a /24 to a /23.

Lets say the network is currently 192.168.3.0/24 and is out of IPs. Static devices are assigned between 3.5-3.99. The DHCP scope is 3.100-253.

Expanding the network to a /23 makes the new range 192.168.2.1-192.168.3.254. Obviously this is not ideal considering the static devices are now in the middle of the range. The default gateway is also set to 192.168.3.3.

If we move the network to a /23 and set the DHCP scope to 192.168.2.1-254 this would leave the gateway as 192.168.3.3. Obviously not ideal but should still work at least in the interim until a full network update has been done.

Using a FortiGate firewall would we also be able to set a secondary DHCP scope in the 3.x range such as 192.168.3.150-250 to allow for additional devices?

Long term we would want to move all the static devices into the lower end of the range, but that will require quite a bit of reconfig for over 100 static devices, printer mappings, etc.

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Alternatively a secondary DHCP network could be setup, such as 192.168.4.0/24 (which would allow for easily moving to /23 if needed in the future) and no existing static devices would need to be modified. I'm thinking this might actually be the easiest to do once I get vendors to update VPN tunnels.

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Any suggestions or thoughts? The goal is to do it with the least amount of effort and cause as little downtime as possible. I'm not against doing a bunch of pre-work to make sure this goes smoothly, but if we can avoid having to update static devices that would be ideal.