I recently installed the separates for all mod, along with separates for every category. And I managed to find something for almost every category via the maxis match repository project. The only thing I’m stuck on is separate athletic bottoms for elder females. I found these ones but not much else. I would prefer maxis match but it's also fine if they're not.
Alternatively, if someone knows good resources for finding separates that would also be very much appreciated. Other than the repository project, I tried googling, pinterest boards for separates (didn’t find many), and various search terms on MTS. But I feel like there are probably better ways out there for finding separates.

Some sporty bottoms for EF that were converted from Sims 3 and Sims 4: 3t2 Roller Sport Shorts, 4t2 Base Game Pants Yoga, 4t2 GP05 Shorts Athletic, and 4t2 Shorts Short Tie.

Oh, I completely forgot about conversions! Thank you so much for linking to so many options!

EDIT: Using the elder tags to search through Rented-Space's downloads I also managed to find these: MDP 4t2 Shorts Loose Long

@Corax Kayleigh83 has some stuff, but no tag for elders. Here's the ones I know of:
https://kayleigh-83.tumblr.com/post...ng-this-preview
https://kayleigh-83.tumblr.com/post...c-bottoms-i-age

These are really great options as well, thanks a lot!

There's a little tutorial on how to categorise clothing here

I use Pick'N'Mix's Outfit Organiser which I find even easier for bulk re-categorising clothing. My problem was that I didn't have a lot of EF bottoms that I could re-categorise to athletic wear.

I see. Yeah, this is a constant problem. I find that DeeDee's Tumblr is a good place for EF clothing, it's worth digging through her age conversions, a couple of things have already been suggested. I had a quick look and found these too: Flirting With Disaster, Yuichen’s February Stars PJ Pants as separates

Those two are great, thank you!
I followed your suggestion and looked through DeeDee’s clothing tag and found a female addon for Skell’s maxis match repository separates. So a perfect fit for the other separates I mentioned finding in the original post, thanks a lot for suggesting it!