We use WebDirect for this. Our main office is in Texas, and we have partners around the world, as far away as Asia.
We built a separate database file just for our partners, using bare minimum layouts and themes. It is really cranked down to just what the partners need in order to submit and retrieve data. We managed to get it down to three layouts with fewer than a dozen fields.
With 13, they found WebDirect performance acceptable, with some complaints about lag. The complaints were mostly related to delays while FileMaker would save the record and update the display… that round-trip lag led to a lot of initial confusion. We addressed the performance primarily through diplomacy ("we're working on it") and training ("take your time, be patient, do one step at a time.")
I've only just moved it to 14, but I am already seeing vastly improved performance, and I'm meeting with folks next week to see how they like it. In my high-latency link simulations, it's about as snappy as I would expect.
14 is much nicer than 13 for this, I can already tell you. Field behaviors are much improved. Load time is much reduced. Page rendering is much snappier. Honestly, where I had been harboring dread that I would eventually have to switch to a PHP solution, now I feel much more confident that I can stick with WebDirect.
So I would look at 14, and I would look at WebDirect. Make something just for your partners that is literally no more than they need, and I'd bet on your success.
Thanks for your experience Jonny. I'm well acquainted with FM 13 and WebDirect. I didn't realise 14 was such an improvement. I'll explore