It will be probably hard for Audi at least in the 1st year.
If' I'm not mistaken they will use their own engine and it will requires time for innovation.
The good thing about it is that they will can boost the progress with frequents updates.
Same thing will be for Redbull, they will build their or own engine from 2026, without the (official) support from Honda.
I also think the same. Building your own engine is a tricky one. Especially if you are new in this sport.

This is just the situation for Audi.
And Red Bull... They are even an energy drink company. I'm really wondering how good their own engine will be.
Probably they'll miss the support by Honda like A LOT!