I humbly request you to add a 5th tier of solar panels and accumulators, and that you make them orange.

Because then, we could fully color-coordinate tiers with rarity.
Common - T1
Uncommon - T2, both green
Rare - T3, both blue
Epic - T4, both purple
Legendary - the T5 i am asking for, both orange

That's a neat coincidence considering the colors are like that for a few years now.
I might consider adding another tier. But it might be too OP even for my taste. Each their own I guess and that's why you can disable tiers.
I might consider only enabling this tier if you have the quality mod enabled. But legendary T5 panels might be way too OP :D

Any name suggestions what might come after ultimate?

Im not sure about that "too OP" thing, legendary T5 (or any T5s really) will be expensive
For the name... I thought about something like "perfect" "(grand?) master" or "mythical", but i think i have a better solution:
If you lock T5 behind Cryogenic Science, you could go with a "supercooled theme". Like, IRL all our superconductors only work at really low temperatures, so it could be called like "Supercooled Solar Panels/Accumulators", and let them only be crafted on Aquilo.
I also considered "Promethium Panels", but then i realized that using promethium in anything other than it's science is kinda not worth it and upcycling promethium for legendary panels also sounds like a big nope.

Edit: Wait that theme only works if Space Age is enabled. Uh. Maybe something more generic after all?