Just took a quick look, but for your mana curve 25 lands are way too little. I don't know how you planned to play that many high mana cost cards. I tried to look at 10 starting hands with the starting hand feature and looked to turn 3 and only one of the 10 hands was what i would call playable. My guess is that you should have between 36-38 lands depending on how your mana curve looks after cutting cards to a the lands. I would also recommend to try to have more lands that enter untapped and not be reliant on having
amulet of vigor to play on curve as only playing tapped lands is like playing one turn behind people that are not.
I also have a
The Ur-Dragon dragon tribal deck and it was my first EDH deck built from
the Ur-dragon precon deck, so i'm very familiar with 5-color Dragon tribal. if you want to have a look here is the deck list:
https://deckstats.net/decks/142156/1429930-might-of-the-ur-dragon-s-brood/I don't know your playstyle or what you want your deck to do, but I kan give some examples of playstyles and how to cheat in your big dragons.
- If you want to get many big dragons out reanimation is a great way. Note:
Scion of the ur-dragon might be a better commander for this. Here
cards like
haunting voyage,
living death and
patriarch's bidding are great cards. And a dragon like
Moonveil Regent can help you put cards in
graveyard.
- If you want to cheat in dragons in an other way, than
Sneak Attack,
Purphoros, Bronze-Blooded,
Selvala's Stampede,
Lurking Predators and
Defence of the Heart are great cards. I also like
Oath of Druids but its a risky card.
- Last style I know is better with
Morophon, the Boundless and it is trying to get to cast your dragons for free by reducing the casting cost with
Morophon, the Boundless,
herald's horn, Ursa's Incubator and other cost reducers. Or
Fist of Suns/
Jodah, Archmage Eternal + Morophon, the
Boundless.
Also you are missing the best card in any EDH deck with
The Ur-Dragon as its commander and that is
Hellkite Courser. 5 mana for a 6/5 and you get your commander out for free with haste without increasing the commander tax. This allows you to use The Ur-dragons attack trigger to Draw cards and put down 1 permanent from hand for free.
If you want some help with suggestion on what to cut for more lands than I would need you to try to explain what you want the deck to do and what cards you are not willing to cut.
Hope some of this was helpful and just remember that not all tips are right for all people as playstyle, power level, local meta and what we like and want out of the game are different.