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I don't wanna get into why one would want/need any of those devices.
It just so happens that I use an OT, an A4 and a modular rack. Both Elektron devices offer unique ways to control a modular. It depends on your modules and what sparks/stifles your creativity on which approach is best for you.
A4 not only offers note CV, gates and sequenced manual cv, but also LFOs and envelopes. All of them plockable. Really cool.
The OT needs a Midi to CV module (like Garfield mentioned). There's no need for an 8 channel interface. You can absolutely use the OT to control just one or two channels. It's hard to recommend a single interface without knowing what should be controlled. Multiple voices using just note cv+gate? Or rather one voice with lots of midi CC to CV? The expert sleepers FH-2 is very flexible, but requires quite a bit of setup time and planning. Something like LPZW's Tram8 offers 8 gates plus 8 CVs (in a special OT optimized firmware), but no note CV at all - might be totally sufficient for a drum rack, or for sequencing eurorack sequencers via OT.