Are there any best ways to get rid of excessive Warframes?

[Vulcax]

I have a few Warframes that are taking up space and preventing me from making new ones. I want to make new slots and instead of simply selling it for 20000 credits so are there any other ways to get rid of it that'll benefit me rather than just credits?

[Wandering Flare]

Once you get a Helminth segment from Son on Deimos, you can feed your frames to it to unlock one of their abilities (predetermined) for use on other frames. That is the best use for unwanted non-Prime frames.

[яᴀʀ 🦖]

at least you'll have an extra warframe slot for warframes you actually want, but yeah you can either subsume it or just take the creds

[Midas]

You definitely want to get the helminth and subsume them. Aside from just unlocking helminth options, it's also one of the main ways to level up your helminth early on.