In any case you'll have to buy something. You have basically two options: (1) mixer, or (2) new audio interface with more inputs and routing options.
If you want to keep your current audio interface, you could use a mixer that has enough inputs for all of your devices, and a possibility to route the signals you want to record into the audio interface, and route the output of your audio interface to the monitor speakers and headphones without routing them to the audio interface's input, which would create a feedback loop. One way to do this is with two "aux" outputs. Each channel you want to be able to record, you route to an aux output. And the audio interface's output, you connect to an input that you do not send to the mixer's aux outputs. This setup can be nice, and sometimes it's handy to have some tactile control over what's happening. And you'll be able to play, mix and change levels etc. without having your computer switched on or even connected at all. But you'll have more stuff on your desk, more cables and everything ... and you won't be able to record all of the devices on separate tracks simultaneously.
A different kind of option is a new audio interface that has more inputs, for example Steinberg UR44 or Focusrite Scarlett 6i6 or something even larger. With a bigger audio interface, you can have all your devices connected and routed to separate tracks in a recording software all the time. Reading your description, this might not be needed really. But then again, if you want to play guitar as well - depending on what kind of guitar it is, you might want to take advantage of an audio interface's "Hi-Z" inputs, which are needed for many electric guitars and basses.
