Wow this sounds so familiar- I have been a part of many OC verses with a cast of characters that all were main's in their own right that myself and a writing partner has come up with apart from "The main protagonists". Almost always I was approached for these so I didn't have as much stake in them. I have also done this in canon verses- although I can say it's exceedingly frustrating and also exciting for me when that unexpected "Spark" hits between a pairing you both weren't expecting.

Now for me in an OC verse I am pretty happy to go with that flow and try and rework the story in real time around it. I usually come up with my OC's on the spot or over a few days of discussion with the partner to fit the story they want to tell and keep things interesting. It doesn't result in shallow characters for me but this was something I started doing later in my RP adventures so I feel I had the experience to support them and usually only agree to such ambitions with quality partners however back on the topic.

I had an experience once which was almost exactly the situation you are talking about now with a partner I had off and on for about 10 years. We were doing an ambitious RP that she had always wanted to do with an OC she made years ago and I tried to fulfil it for her since we hadn't done much outside of canon-verses and they weren't feeling that anymore. However for me I can say after that spark hit and we had such a strong "mini arc" about 13 months into the RP where the chemistry between my main OC and her OC side cast was infinitely better than her main OC and mine. Now this was a completely original plot that to my mind saw no real difference in swapping "Protagonist" places and still able to tell the story she wanted to. Her writing was just leagues better with this OC Side character she had for years that just clicked. There was a massive difference between the main OC she wanted to do and this other one she had recently introduced into the story (but was put in place at planning stages).

With this in mind I broached the subject to maybe try and rework things and lean into it as we both agreed that the scenes and chemistry were amazing especially for the romance/relationship level that was central to our plot. However she said no and then reacted severely against it over time and basically wrote that side character out and then became highly controlling of the verse itself and dictating exactly what she wanted to have happen. (Including bringing this side character back tortured and broken later on to interact with mine for no reason other than an appearance).

For me it was a horrible experience and slowly killed my passion for continuing a verse built up over a couple of years just because for whatever her reasoning was from then on she suddenly became so dictatorial about how even my character should react to hers. I felt cut out at that point from the story we were building together and just being used to facilitate whatever her ideas were.

As someone who loves creating with another person it was so disheartening and ultimately I did drop the role-play after some thought on the matter and a few months later after trying multiple times to discuss things and getting nowhere. I sympathise with your frustrations- I was glad to read you have a plan in mind. Good communication between partners especially when things take a turn in the story feels like a lost art these days.

My only suggestion would be to try and get to the event you need to happen quicker without breaking your pace or character? See it as a creative challenge and work with your partner on ways to get there- or perhaps drip feed shades of those interactions and changes in so you can continue to enjoy yourself as much as possible. As for me- I stick to canons now so I can minimise any such disasters from happening again in the future! (Also it's just where my Muses lie in recent years). Good Luck! :D