4k + 5k footage, 1080p render, what timeline size?

Hi all. I'm making a short video using 4k and 5k footage and I'm punching in quite a bit to 2x and 3x at times. I ultimately want to render good 1080p video.

What settings are optimal for this? Currently using 1080p timeline, and I upscaled the clips 2x in attributtes but wondering if I'm losing quality on the final 1080p render in those sections where I punch in to 2x and 3x, because my first render those punched in sections don't look all that great.

Will changing the timeline to UHD next time give me a better 1080p render in this type video?

Thanks for any input.

Nope. Working in a 1080 timeline for 1080 delivery is perfectly fine.
If you are punching beyond 2x (for 4K sources), are you using SuperScale? It should look fine up to 3x.

If not, what's your source? If they are so-called "4K" from a sensor with 4K photosites and massive compression like H.264 or 265, don't expect miracles. Can you post an example?