In fact, it should be RELATIVELY simple to set up a suitable configuration that does roughly what you want.

If the Elektron units are not being sequenced by each other, you can switch off their response to incoming MIDI (except clock) in the Project or Global settings menus.

Also, you don’t have too much extra kit, and most of the units only require one MIDI channel each. You could almost chain it all on one MIDI port of the Unitor but instead, for maximum flexibility with moderate amounts of repatching of the Unitor, here is what I suggest for MIDI connections:

XS Out to OT In for XS to play into the OT sequencer.
OT Out to MM In for OT sequencer to control MM tempo.

MM Out to Unitor In2 for MM sequencer to control other gear if desired.
MM Thru to MD In for OT sequencer to control MD tempo.

MD Out to Unitor In3 for MD sequencer to control other gear if desired.
MD Thru to Unitor In4 for OT sequencer to control other gear if desired.

Unitor Out1 to XS In for control of XS and iPad if desired.

[Optional: OT Thru to Unitor In1 can be connected later for other purposes if desired.]

Unitor In/Out5 to AJ Out/In.
Set AJ to channel 1.

Unitor In/Out6 to TX Out/In.
Set TX to use channels 2-6.

Unitor In/Out7 to K4R Out/In.
Set K4R Multi & Drum to use channels 7-12.

Unitor Out8 to XD5 In.
Set XD5 to channel 13.

XD5 Thru to SQ In.
Set SQ to channel 14.

SQ Thru to RSP In.
Set RSP to channel 15.

RSP Out/Thru to KP3 In.
Set KP3 to channel 16.

You would have the choice of taking MIDI Out from one of XD5, SQ, RSP, or KP3 back into Unitor In8 as needed.

This way you get to send output from OT, MM, or MD anywhere you like because you have each of their MIDI outputs connected to all other Unitor ports; you can select which instrument to send to simply by selecting MIDI channel.

Before you start, you might prefer to use more channels on either TX81Z or K4R which would require either sacrificing a channel on one of the other units or setting up a new configuration for the Unitor using appropriate internal routing.

You could even use a channel for the Unitor itself so that you could control its internal configuration from one of the sequencers.

When you add more gear, you can chain the new items to the gear on the existing ports, although that will mean that you might need to: reduce channels used for existing gear, do more patching inside the Unitor, or audio mute output from gear that runs on the same channel.