I'm running a short campaign for some beginner players, a few of whom brought their partners to join in the latter sessions, and ended up with 9 PCs (level 5). Their final encounter is going to be a dragon fight. CR calculator doesn't really help because it recommends harder dragons who could TPK the party with a single use of breath weapon, and I have a hard time balancing encounters as it is since it's my first time DMing. I was thinking of this one (https://pin.it/5Wly7oMLB), but lowering the DCs and adding lair actions, but I'm still unsure. Does anyone have any tips?

You're going to want something with an effective CR of 10 or so, but it's true that a CR 10 young red dragon will oneshot a level 5 PC on a failed save. This is not necessarily a showstopper, mass healing word is available by this level, but it does make an extremely swingy fight because if the wrong PCs fail their saves people just lose. If you want to get a more consistent battle, I suggest just taking that same dragon and

1. Reduce breath weapon to 10d6 (35) (-21 damage; the way CR is computed, this is treated as losing 14 dpr)
2. Give it one extra attack per turn, at damage similar to the claw -- say, a bonus action tail attack, or possibly one legendary action per turn.