To my knowledge, IS Boxer is the only software option where you Interact and DPS with a single key.
It is still two keypresses for two actions.

You can create a Keymap with two steps.
Step 1 - Send IWT to all clients.
Step 2 - Send the keybind to all clients.

Then just rapidly double click your dps keys.

Otherwise, you'll need to have an IWT key, which is spammed every time your toons are not close to the target, and then other sets of keys for whatever DPS you are doing.

If you spam the IWT key, your toons will circle around the target, which means half of their attacks will be from behind, which is a benefit for you. However, as mentioned, if the key is spammed while the target is dead, your toons can run off into the sunset; you'll want to have a follow key ready, or strobe follow (outside of combat) enabled.

Also as mentioned, enable the "Click to Move" option, otherwise IWT is pretty useless. The default action is to use the NPC or attack a hostile creature. But without Click to Move, positioning/distance is an issue. Most people seem to really dislike Click to Move, on their active toon, but its a great tool on the slaves. If you can stand having it on the active toon, then you can lead your team from any toon, and all of the others can IWT effectively.