Need help configuring my two routers while avoiding double NAT

Hi there!

Forgive me if any of this sounds stupid, I'm pretty new here and most resources tend to be confusing for someone like me.

Let me start by explaining my current setup:

One router/modem combo, ISP provided. Then there's a router in my room running OpenWrt (with DHCP disabled), they're connected by LAN ports. Seeing as I am a teenager (which seems sorta rare in this space lol), I don't live alone, and I can't buy another router. Also can't flash the main one with OpenWrt.

I want my own router to handle routing and DHCP for my own devices, while leaving the rest of the household alone. Our connection isn't the greatest, and I'd like to avoid relying on the crappy ISP device for most things. I'd rather not split the network up too much either though; I need devices to be able to communicate.

I'm wondering how I should be setting up the devices, what would be optimal for this situation?

Bonus question: I lied, I have two router/modem combo devices from two separate ISPs, we have two different internet plans (don't ask, long story). Sometimes I switch between them depending on my needs. Basically, this isn't needed, but does anyone know if there's some way I could make it more seamless and maybe connect the networks a bit somehow?

Thank you!

EDIT: About that last bit, it's just a secondary thing, the rest is more important. I was thinking of just using an unmanaged switch to be able to connect my openwrt router to both at once, but wanted this sub's opinion too.