I am so very sorry for your loss, and excited for your new pregnancy. I appreciate everyone's pain and experience is different, but I hope you don't mind me drawing on my own one in writing this.
I lost a baby this winter at 23 weeks pregnancy, and am now TTC again. I loved our baby who died so much. They were so wanted, and I was ecstatic being pregnant and to start our own family. I gave birth to them, held them, named them, etc. They feel part of our family, and I somehow need them in the fold, and be connected to their future siblings. I have been struggling to give them a place that honours them and shows I will never forget them. In the beginning I was almost making our home into a shrine to express that feeling, and I am now learning that I do not need the 'props' for our baby to know that they are loved, and to remember them. Our first baby will always be part of us, our future children's sibling, etc., and that fact isn't based on what I do.
I hence understand how tempting it is, but a new baby deserves their very own, unique name, and their own initials, as being a rainbow baby is hard enough. I am already wondering how my experience and acute awareness of the fragility of life will affect my future parenting of them. How the grief I have experienced will influence my engagement with a future baby, etc.
Even if you don't plan to emphasise the fact your little boy is a rainbow baby, as you mention, the fact that commemorating his sister is a consideration in naming him clearly shows that you struggle to steer clear of this. When he is older, and knows his family's history, he will realise the rationale behind his name, and you can't predict how he will feel about it. Knowing the heavy meaning behind it, he will never be able to challenge or change his name without feeling like he is hurting you.
If I am honest, I think I owe it to any future babies to let them be completely and utterly unburdened by my personal pain and emotional needs. To let them be their own person by having their own name, with their own initials and own unique meaning. To let them find their own way to incorporate their older siblings's memory into their lives on their own terms.