If you’re looking for non IRIS keypads which work with SmartThings you can get some links on the first post of this topic.
However the IRIS 3405-L should work just fine, including the built in 65dB alarm with the above DTH. When used with a SmartApp like LUM you can create users for your keypads, custom actions and lots of other things including arm/disarm Classic SHM. These apps require access to RBoy Apps.
As for copy pasting code for custom DTH’s, it’s a fairly simple process. Here’s a step by step instruction. It’s written for RBoy Apps but it applies to any generic DTH
You can absolutely use ST without a keypad but since you have a keypad you can lots of wonderful things with it, it can be used a panic trigger, it can be used as a second keypad for your locks, it can be used to arm/disarm your system, it can be used to create custom actions.
Just for reference I’ve got 6 keypads across the house which range from opening/closing garage/patios doors, arm/disarming systems, running custom actions, panic buttons, using them as a Siren/Alarm, using them a chime/beeper to be notified when doors are opened/closed and lots more. It’s just a very powerful option to have.
For chiming the keypads when sensors are triggered, you can check out this new app: