Block unnecessary traffic by counrty

Many firewalls with active subscription has database of IP by country so that you can block by country.

I actually permit only my country, and block everything else by default. 

Okay, I do allow SMTP inbound before I block everything else that isn't my country. One could argue that you should block Russian and North Korean mail anyway.  ;-)

If I went with the z4w Untangle appliance would that sit outside my Cisco RV340 or replace it?

A firewall could conceivably be in front of, behind, or replace the RV340. Kind of depends on what the RV340 is going.
Firewalls replaced my routers.

I love the Untangle solution and it seems to give me some bang for my buck but I would do a disservice if I didn't ask if there were others comparable options that any of you have also tried?

On the Untangle z4 Plus what is the difference between:

My only concern from what I saw looking at it is the throughput net speed.
 

Take an older PC ready to be replaced with something newer.  Install two identical network cards in the machine.  Load any kind of linux - Debian is my favorite.  Install xtables-addons and then subscribe to a geoIP blocking list.

Then for an "accept" setup:

# RULES BEGIN ============================================

# ==== CA -- Canada
$iptloc -t filter -A $chname -p tcp -m geoip --src-cc CA -j RETURN

# ==== GB - Great Britain
$iptloc -t filter -A $chname -p tcp -m geoip --src-cc GB -j RETURN

# ==== US -- USA
$iptloc -t filter -A $chname -p tcp -m geoip --src-cc US -j RETURN

# If here, no accept was matched.  Reject the packet.
$iptloc -t filter -A $chname -j REJECT

# RULES END ============================================

For a "reject" setup, change the IANA country code as necessary, change RETURN to REJECT, and fall through at the end instead of rejecting.  The disadvantage of a "reject" setup is that there are so many country codes that are undesirable, and the list goes on and on.

Note that more rules are required to select the incoming interface and forward the traffic from interface A to interface B.  This is an illustration, not a working example.

I consider this to be much easier to maintain than a special-purpose firewall / router.

That said you will have to trust the data supplier about what country has the the GEO location of an endpoint.
I happen to have an address that was 2 years ago belonging to a CZ based hoster.  Other customers from the SAME ISP complain with various provieders of services (Sonos f.e.) that still things the address belongs to  CZ or even Dubai  where it is clearly NL. which is advertised by the ISP as such. (and actively persued as af as possible).

Be sure you get up to date databases or database services, address/ranges DO migrate accross borders. (and may be even advertised in different parts then they are actually used VPN anyone?).