I’ll just answer what I can speak for. I’ve only had my OB-4 for a few months now. I love it but based on your questions I’m not sure if it’s your best bet.
Is the unit sturdy and portable enough?
Portable, sort of. I love carrying it around my place but I’d need a protective case before I’d feel comfortable bringing it outside.
Sturdy, no. Gets smudged easily and I’m constantly worried about knocking it over. Which has happened a couple times.
Is the Line-In a reliable option?
Yes, and they improved the latency in a firmware upgrade. The only issue is there’s some sort of gate and it can cut off the first part of the sound it receives if it’s on standby or something.
you can make loops but can’t export it directly?
Correct, and honestly I’ve found the looping to be really challenging. It’s not terribly easy to adjust the start point, I don’t think you can adjust the end point. and the scrub wheel thingy moves fairly slowly.
I was wondering if one could make some loops and then record them onto the op1 Field via FM. Is that possible?
No, the OB-4 can’t transmit FM. I do often play into the OB-4 via an FM adapter (which I suppose the OP-1F already has) which is fun but very lofi.
and what about the other options? the mantras, the noise, the metronome…they all sound a bit too gimmicky. Are they of any real use?
The short answer is “not really” and the long answer will make me sound like a TE shill so brace yourself.
I think the design is very intentional: there’s no line out because they made a speaker and only want you to use it as a speaker. I understand the frustration but I think they were aiming for a specific “experience” and I’ve actually really enjoyed it.
If you’re looking for a speaker with a lot of qualities that extend what it can do you really ought to look elsewhere, but if you’re looking for a speaker that somehow can keep you curious about it for months, it’s a good choice. I was looking for the latter–something to pique my interest for a while–so I really dig it.
For example, most recently I’ve started using my OP-Z’s mic to record the OB-4’s output into the OP-1’s tape. Pointing the mic at different parts of the speaker yields different results which can then be layered. One day I’d like to use the mantra to modulate a synth on the OP-1. Plus now I listen to the radio way more which is great.
Of course, all of this can be done with another device but the OB-4’s limitations pushed me this specific way and it’s been a blast.