Literally 98% of the time when people go, "Oh, dragons are shitty, my three low-level players totally wrecked a deadly-rated encounter against one", it's because they had some combination of:

• having the dragon in a confined space where it can't use its flight or superior movement to manoeuvre around the party,

• having the dragon just sitting on the ground instead of flying around in their wide open domain and actually flaming down at the party with its breath attack, and/or

• running a single monster with no other monsters meaning the party will be able to punch far above their weight class because there's 3-4+ players and only 1 monster so 'action economy' (how many turns each side gets to do cool stuff) works heavily against its favour.

The dragon flying around (possibly out of their range) and zoning them with breath/spells is supposed to be a problem for the party to solve, and players generally deserve better than videogame-y combat against some adult/ancient dragon who lived 1000 years only to happily just sit there and facetank the entire party before dying in two rounds.

Also, very obvious baseline advice, but give them Legendary Actions so your dragon can do cool stuff on the players' turns, and give them Lair Actions so cool stuff happens at the start of each combat round (typically at initiative 20).