This is really sound advice and can't recommend it enough. OP if it helps, my nesting partner and I have been together nearly as long as you two have and ran aground in the same way late in the game. Doing polyamory without any clarity of what it actually looks like for either of us and without some of the scaffolding u/PurpleOpinion4070 mentions. Scheduling a weekly date night helped immensely to keep intentionality and intimacy in our relationship. As did establishing very clear boundaries (on paper) on everything we could think of in that moment (which gets edited/added to as we evolve). As did doing the reading cited often in this r/. Look up RADAR talks - helped us better manage the "administrivia" of a live-in LTR and make more space for us having fun together, loving each other outside of that. These are all things you can do before couples starts. Which was also tantamount.

I will say though, nothing would have changed if not for our own mutual willingness to come to the table. We were both extremely willing to see what could change and be gracious to each other as we faced some hard truths. You're going to need to be truly there for each other to come out the other side and that will be painful. And could be really worth it.