Thanks for the invitation. I could go on for days about his design but I'll try to start with some reasoning around how we ended up where we did.
Off the bat your instincts are correct Aphelios' design had to be simple to support the weapons he carries as they need to be fairly clear at all times. I tried to fuse the robes of a religious priest with the classic look of a secret agent (tactical turtleneck!) to create a clear aesthetic that drives his theme. Comparing him to Diana I actually tried to carefully replicate the same materials seen in her design, but arranged very differently. She is a Rakkor raised by the Solari who found the ancient armour of the aspect of the moon - a warrior-avatar of the forgotten Lunari sect. By contrast Aphelios is a monk of sorts, equal parts minimalist utilitarian (the black suit) and divine ritualist (the holy robes). He's not dressed to take a hit, he's dressed to perform a ceremony. He is also dressed in the way of the modern Lunari which have likely made changes particularly because they are few and living in hiding. When they walk amongst the Rakkor they must hide their true selves so when they are free to wear what they want rich forbidden colours are on display. Were Diana welcomed to the modern Lunari I could see them dressing her similarly to Alune for a religious event, celebrating the return of their scion. I'll also just point out that LoL champions represent the most extraordinary characters in the world of Runeterra. They are the ones that really stand out from a background of fantasy tropes. LoR is a great example of what the rest of the world look like but in LoL we are focused on the exceptions to the rules.
The weapons were a much longer journey, being designed and redesigned over and over. Originally they were wholly moonsilver like Diana's blade. Diana's blade seems to be able to launch out beams of moonlight, so it's not at all a stretch to see that honed into a projectile weapon, but Aphelios is so different to the rest of the Lunari (and the Rakkor in general) we felt it needed a unique material. Something rare and potentially fragile, something only wielded by him, held and empowered by his sister's magic, something once enervated would be useless to allies and enemies alike. Shaping the moonstone was then a careful balance between the Lunari theme and the gameplay requirements. For each weapon I imagined how he would hold it and tried to connect them to weapons other marksman wield as a way to ensure that if you play LoL, playing as or against Aphelios you'd immediately have some idea of what was going to happen next (oh it's like Caitlyn's rifle, oh it's like Gnar's boomberang etc). Every day I'd have people come by and I'd ask them to guess what each weapon did until I was reliably getting correct answers. On top of that each one was tied to a celestial phenomenon - this served to further reinforce gameplay (Black hole has a CC effect) and give them a colour identity to support gameplay learning and recognition. I imagined the Lunari sort of like the Eldar in 40k, sculpting these smooth graceful shapes inspired by the phases of the moon, swooping crescents and curves of the phases, orbits of the moon, flow of the tides. The one additional element, a flourish added when they were weaponised, the carved ridges present on 4 of the 5 that mimic the shape of the flower 'moonbloom' from which Aphelios' tonic is drawn.
The weapons are very sci-fi, his whole aesthetic is quite future-fantasy, but that is a side of Mt Targon; at the base you have the hardy and Hellenistic Rakkor, but the higher you go up the mountain the more alien things become until you reach the top and touch a wholly unnatural space. Each character has different amounts of these two elements but bringing them together is what sets Mt Targon apart from other fantasy locales.
