I was counting on the bard and the cleric to be providing buffs. After all, [MENTION=5834]Celtavian[/MENTION] has been pretty insistent that you can get advantage/buffs/whatever, to such high points by 8th level that the -5 from Sharpshooter/GWF becomes meaningless, so, I'd assume that the effective -5 for disadvantage would be similarly easily overcome.
For it to start its round at 60 feet to breathe, means that the dragon has eaten at least 4 attacks already. It had to. Remember, our fighters do have a range of 120 feet with javelins. Plus a 30 foot movement. Any round the dragon breathes, it's going to eat 4 javelins, possibly 6 with Action Surge. Or, 6 every round with a simple Haste spell. I'm not saying the fighters can do it on their own. I'm saying that they don't need to fly to do it either. At some point, the dragon's going to want to bring down it's big damaging attacks - and that means melee.
It just occurred to me that this statement isn't true. Dragons have an 80' move and a legendary action for 40', so they can start 60' away from the party, use their breath weapon, and be back at 180' by the time the following PC has moved simply by abusing the predictability of non-speed factor initiative. Here's how it works, assuming the dragon has already observed the party's initiative order:
Dragon starts at 180' on round N, out of range of javelins/fireballs/lightning bolts/etc. (A Sharpshooter or archer skeleton platoon would pwn him here but the hypothetical melee-heavy party hasn't got any. Their loss.) Assume initiative order PC #1, #2, #3, #4, Dragon, because the dragon can just mentally designate whoever goes right before him "PC #4". Dragon's action on round N is, "Hold until PC #4 does something, then move to 100' away from nearest PC." It will close the distance to 100' on PC #4's turn, and use its legendary action at the end of that turn to move to 60' away.
The on the dragon's own turn, it breathes on the nearest PC and retreats to 140'. At the end of PC #1's turn it uses its legendary action to move back to 180'.
Result: PC #4 gets to engage the dragon once at 100', and PC #1 gets to engage it at 140'. (Distances here refer to "distance from nearest PC", not necessarily "distance from me".) Each of them can spend movement to reduce that distance by e.g. 30' (standard movement in good terrain), so PC #1 could probably throw a javelin. Everyone else is out of luck unless they can engage at 180' (150' with movement).
If the dragon does this, not only do the javelin attacks have to be made at long range, but only one PC even gets to make them! And if the evoker isn't PC #1 or #4, he doesn't even get to chuck a Fireball unless he happens to be the PC closest to the dragon, in which case he is probably down and dying on the first attack and every attack thereafter (and therefore doesn't get to cast his spell anyway[1] since even if healed he has to spend 15' of movement standing back up).
Yeah, this party is so toast against a dragon if they try to approach it straightforwardly. I would Darwinize them in an instant. They'd be so much better off getting a druid/ranger/shadow monk and approaching the dragon under Pass Without Trace at night--they might get a surprise round then as well as free pass into melee range. Other options include "occupying the treasure hoard with Leomund's Tiny Hut while the dragon is away", which probably results in it doing something or other that brings it into melee range as it tries to take the treasure back/bury you under ice/etc[2].
[1] Unless one of the other party members does something smart like putting the evoker on his back and Dashing for 60'.
[2] BTW, D&D dragon hoards are pathetically small compared to Smaug's horde. 130,000 gp is barely over a ton of gold, which is 0.135 cubic meters. The dragon can't even turn that into a pillow, let alone crawl inside it for a nap.