Does this fight feel fair given my group?
So, I am DMing a group through Tyranny (using 5.5e), and I am adding in side quests and such to flesh out sections and just make my own. We started with 4 players initially, but one dropped out due to out of game responsibilities. I have repurposed an area/quest that was intended for where he would have gotten his custom leveling item (Using Ancestral Weapons form DMsGuild to let them have something that grows with them, 2 players have theirs and 3rd getting theirs in Elturel, just common adding a cantrip so far). It is a temple that they got a side quest sending them to from a town they stop at between Greenest and Elturel.
When they attacked the Hatchery, they ran into Cyanwrath first, and playing off the adventure saying he is more loyal to Frulam than to the Order, had him yell in draconic when near death for Frulam to escape, the cave is lost (set up later that Rezmir had okayed abandoning the eggs to prevent capture, as having Rezmir believe the adventures are part of a more organized resistance and so doesn't want information spilling out). So Frulam and her guards bug out.
The Side quest is that the PCs were hired by the innkeeper of small town (if you're in the US, think those small towns that spring up by Highways that are just a hotel or two, and a couple fast food restaurants, and not much else), under pretense that a pack of wolves being led by an owlbear have become more bloodthirsty, having attacked the town and killed a resident and cows, and even attacked a trade caravan. In reality, Frulam arrived, killed the resident for resisting, kidnapped the towns kids (innkeeper just said they were being kept inside when PC asked) and told the town to send any adventurers that come through town to the temple on the "quest".
I decided to set up that maybe Frulam is a Death Cleric (to me seems logical that a Cleric of Tiamat could be Death Domain, given Cults old methods) and that while she is on board with the new plans, used to help with the old undead approach so has some spells and spell scrolls. For narrative reasons custom to our campaign and pulling from our previous campaign, there was fighting at this temple and a dead young green dragon.
PCs are all lvl 5, with an Assassin Rogue, Life Cleric, and 4 Genie Paladin/1 Warlock. Cleric this is their 2nd campaign (other than some back in the 80s, rogue was back in 80s some as well, and paladin was 3.5). This is my 2nd campaign as a DM (recently started playing in another one for first time) so still getting a hang of balancing, and trying to account for how party comp effects how reliable CR is.
On arriving to the temple, they had a first encounter of 6 zombies and 3 ghouls. The Cleric did use Turn Undead (only used 1 Channel Divinity Charge, so still has rest) and that trivialized the fight with 2 ghouls and 3 zombies failing their checks. Rogue got beat up a bit, used a health potion so is still down a few HP. P/W used 1 spell slot, not sure if pact or regular, but not time for rest anyway. Frulam called them into temple, tried to interrogate them, realized they were lying, and so decided to just "question them when dead". She then raised the dead dragon (using a Young Zombie Dragon stat block I found on SRD website) and 4 more zombies, to back Frulam (using the Tyranny statblock, but had planned to up her health till started worrying about if I made this fight unfair) and 2 guards (Had planned Dragonclaw statblock, but wondering if needing downgrade).
A Moderate encounter has an xp budget of 2250, and deadly 3300, this falls inbetween at 2600 if use Dragonclaws, 2500 if guards. But I am wondering if I have failed to account for something. Obviously the Cleric can use Turn Undead again, and potentially trivialize the Dragon, but if it passes its save, I am wondering if I have failed to account for this group. As a just in case I have set up a narrative note that the cleric could tell whoever the diety associated with this temple was (Selune) was still paying attention to it in a small way, and can give them the ability a few turns in to pull a few things from them (thinking a heal, couple spiritual weapons, that sort of thing). But just keep second guessing myself.
Thanks for any insights!
TLDR: Using 5.5e rules, is a Zombie Dragon, Frulam Mondath, 4 zombies, and 2 Dragonclaws a balanced difficult encounter for 3 PCs at lvl 5: Assassin Rogue, Life Cleric, 4 Genie Paladin/Warlock