How do I ask my bf for an open relationship?

TL:DR - how do I ask my bf of 5 years to move from being monogamous to being open? What tips do you have for that initial convo?

I (m40) have been in a relationship with my boyfriend/partner (m39) for nearly 5 years and lived together for 3 It’s my first serious long term relationship. We agreed early on that we would be monogamous and I have been faithful the entire time (and believe he has too).

At the beginning, our sex life was fantastic. 3-4 times a week, very satisfying. Unsurprisingly, as the relationship progresses, it has dropped off a bit (more like once a week or slightly less) and become less satisfying and adventurous. We still have sex and I still enjoy it, but I miss the variety of sex with others and find myself longing and sometimes fantasizing about it.

To be clear, I love and respect my partner very deeply. I see him as my partner in life, my equal in our relationship, and my most trusted best friend. I have no desire to hurt him.

But, the sex we have now isn’t satisfying enough. Also, frankly, I miss the excitement and passion of hooking up with other people like I did when I was single. I don’t have any desire to bring anyone else into our relationship or diminish my love and connection with my partner, but i need more and more varied sex and partners than I can find with one person. I suspect I will always feel this way and sometimes I feel resentful or limited not being able to act on it. It’s not that my partner isn’t good in bed, it’s just that I miss being able to explore sex with freedom.

I’d like to propose some kind of ethical non-monogamy, whereby we can have sex with other people, with clear rules, good communication, and without diminishing the love and connection my partner and I have in other areas of our relationship.

I guess I’m looking for ways to start this conversation. How do you go about proposing it? What do you say?

I’m worried my partner, who is anxious and often needs a fair bit of reassurance about things, will take this badly. And part of me is a bit worried that if we do open our relationship, it will sap the affection and sex drive we still have for each other. How do you remain intimate and connected with someone while still having sex with other people.

Lastly, I’m not so much looking for advice about how to have an ethically non-monogamous relationship. More looking for how to manage the initial conversation and how/when to bring it up. Many thanks for any advice.