Your G3 550w will be fine, no need to upgrade, its a great psu.
https://www.anandtech.com/show/14618/the-amd-radeon-rx-5700-xt-rx-5700-review/15
Total system power consumption with a 9900k and 5700xt is less than 300w even on furmark.
Yeah and even then it doesnt seem to affect performance at all over just letting precision boost do its thing.
No it doesn't void warranty unless you do something especially stupid.
Your temps are fine.
Nothing wrong with your idle temps either, both are fine.
Your GPU temps will have a higher idle as most cards these days just turn the fans off when under 60'c.