Thanks for the thoughtful words and it applies to me; a newbie planning a summer cross-country ride. Four panniers will be used, so I should have plenty of space for extreme weather gear.
Do you think my Big Agnes Copper Spur Bikepacking tent will hold up in pounding rain, or should I take a tarp as well? If a tarp is recommended, how does one use it?
Although this question is directed to someone else, I'll add my few penceworth.
Will your tent be good enough? Probably. (I don't know the specs of your tent). But the waterproofness of your tent is only one factor.
Where and how it is pitched is important. Will water be running downhill or pooling around your tent?
The fly material is very relevant - some sag in cool temps or wet weather which means water transfer from outside to inside if they touch.
Is there a gap between the ground and the fly? Angled rain can "bounce" inside.
Breaking down a wet tent is very different to when it's dry. That's an important skill too, to prevent damage that might mean a leak the next rainy night.
And lots more.
Even with the best designed tents, there is still an issue with getting in and getting out - the perfect time for water to get inside.
There is a tendency, especially when we're starting out, to buy the best gear for every situation when often we're not sure how to use what we have properly.
Remember, people have been travelling all around the world for millennia with none of our modern gear!
The best answer to that, I have found is to practise! One night of experience in a tent on a foul night is worth days on the internet. If you have a place at home to pitch a tent then do so on windy days, on wet days, on stormy days. Or a friend's place!
The other part is not to expect perfection. On a long trip things will go wrong, or at least not be perfect. Next day, you'll have a bit more knowledge!
In my experience, the mental equipment to deal with things is more important than the physical equipment.
On the other hand, there are few things as satisfying on some kind of a primitive level as racing to get your home built before heavy rain, climbing inside and being warm and dry
Keeping an eye on the weather is useful. WindyApp is a useful online weather watcher. Start using it now and get familiar with it, or whatever tool you might like to use.
And never forget that unless you're really, really far from civilisation people are around and very helpful.
So, to summarise, I'd suggest you get as familiar with your gear as much as possible, stress test it, so to speak, before trying to acquire new gear and the appropriate skills.
If you're not aware, CrazyGuyOnABike.com is a very useful site for inspiration and research. You can even search tour journals by "disaster". (Just stay away from the fora - they're toxic!)
Best of luck!