You could add a few user-agent strings to your Firewall Rule, but using UA in a security rule is by nature faulty, as one can easily forge their user agent. Also, differentiating a mobile browser from a webview app is close to impossible, as developers may or may not use the default webview user agent when they code their apps. Searching stackoverflow.com for “android vs webview” may help you.
If still you want to combine your country restriction (a Firewall Rule, I presume) with mobile user agents, then if your Cloudflare Plan is Business or Enterprise, you can use regex to match terms from mobile and/or webview user agents, and this example from Cache by Device Type could serve as inspiration.
Otherwise, you can to create a Firewall Rule with several terms, linked with AND logical operator.
If Country is not in {"US" "UK"} AND
User Agent does not contain "mobile" AND
User Agent does not contain "ios" AND...
Start by familiarizing yourself with the documentation and practice with the provided examples.
It’s up to you. Combining several into one rule has the advantage of using fewer rules, and if you are on Cloudflare Free Plan, you can only set 5 Firewall Rules.
On the other hand, many expressions mixed in the same rule make it more difficult to understand and debut, should there be a problem down the road.
You could use the rule you created above and add terms from mobile user agents, but again, I don’t think it’s the best idea to count on UA as a defining element of a rule. It could be used in combination with other elements, but not your your case, I’m afraid.