And what, you're going to ready an action to dive behind cover when you expect the dragon is going to breathe on you? If you're going to spend the whole time cowering in the background, instead of up in the dragon's face, then you might as well not show up. Remember that your Dex modifier is -1, so you're not doing anything with a bow.
It's not feasible to add new party members, as preparation. You can certainly choose for your existing divine spellcaster to cast this spell, rather than something else, but it takes their action and concentration.
As above, but in terms of action economy, the divine spellcaster in your party is probably going to cast Bless instead of Resistance.
Which is actually the point of this whole tangent. Without a paladin's aura, high-end saves become entirely unmake-able without preparation. With a paladin's aura, sufficient preparation can make saving throws make-able, though not reliably.
Just because an item is Uncommon, does not mean it's for sale, or easily find-able. You might have time to go on a quest for one of these, if the DM is feeling amenable, but more than that is unlikely. This isn't third edition.
Like I said, you don't get to recruit party members in preparation for a fight. You could very well choose to have your wizard caste Haste on the fighter, though.
Summing up the feasible precautions you can take (-1 +2.5 +1 +Advantage) gets your final roll at +2.5, with Advantage. The fighter gets two tries to hit DC 21, rolling d20+d4. Even if you can convince the dragon to fight you in terrain that's full of three-quarters cover, and it somehow can't negotiate a clear firing line to get around that, a +5 bonus doesn't shift you into probable territory. You're still doing a lot of work, eating up actions and concentration for half of the party, in order to get less than an even chance of preventing 45 damage.
Valid points on both sides for an occasional issue. You don't often come across saves that high, they will only be difficult saves for certain group members, they often have an inconveniencing effect or an effect that is temporary, or just hp loss. Some group members may have magic to remove effects, limit effects, or boost saves. Some classes have class features that can do the same.
While I agree, natural 20 should save, I think the rest of the issues are largely intentional.