Well, I have a couple "answers" – an answer to the question you're not asking
& a direct answer to the question you are.
First, the question not being asked is about how I would approach your specific scenario – shadow dragon knocked prone by summoned dire wolves, pinned beneath folding boat, and sniped to death resulting in anti-climax? One of the strengths of D&D is empowering weird player solutions like this – I believe we want to encourage that, rather than reign in it.
There are two details worth addressing: (1) knocking the dragon prone causing it to fall (as I understand your example) & (2) pinning it with the folding boat.
(1) While knocking flying creatures prone causing them to fall is entirely RAW, there's good reason to be cautious of this rule exactly for encouraging these kinds of "loophole" player tactics. If that's enjoyable and desirable for you as GM (sounds like it was enjoyable for your players, so that's not at question), then let it lie. If not, or if through speaking with your players you can imagine a better outcome, that's definitely what I consider a weakpoint in 5e's rules that's worth house ruling. You didn't ask, but there you go.
(2) Pinning a creature under a folding boat is, of course, completely outside of RAW and is up to GM discretion. In this case, I would have employed a "yes, but" ruling utilizing some or all of these caveats:
- The folding boat bestows the Damage Threshold 15 of a sailing ship upon the shadow dragon covered by it, and any attack missing or failing to deal damage (after deducting Damage Threshold) against the shadow dragon instead damages the folding boat.
- Establish a clock or Legendary Action allowing the shadow dragon to worm its way through the broken under-hull of the ship.
- Give the players 1 round, then on the shadow dragon's next turn allow it to speak the first command word to return it to an itty-bitty box, and work how it knows the command word into the overarching narrative.
- Give the players a few attacks, then cause the weight of the sailing ship to crush down on the shadow dragon, breaking into an underground chamber that drops the shadow dragon and the PCs down into it, with the creaking ship partially breached through what's now the ceiling above.
I wanted to answer what wasn't being asked
first because I think those factors are critical to understanding why this pitched battle isn't the best representation of how solo monsters are imbalanced against large parties. Because hit points / damage weren't the primary factors, rather non-standard tactics and GM adjudication were the deciding factors.
That said, RAW solo monsters in 5e are imbalanced against large parties.
Now to your direct question: Possibly, but it is a quick hack solution that misses several key issues including stun-locking.
Generally speaking, there are 3 critical aspects of a boss fight (with a specific eye towards facing large parties):
1. The boss needs to be able to threaten about every PC in the party consistently, round after round. If it cannot use a multi-target effect like Breath Weapon then it needs to make that up in some other way to be threatening all 7 PCs round after round. How you modify the monster to do that is up to your GMing brilliance, but that's the principle.
Which is why I gave your "two initiative turns" proposal a lukewarm maybe,
as if the same tactics were employed (stun-lock + snipe) against the RAW shadow dragon just with two turns, those 2 turns wouldn't do bupkiss for it.
2. The boss needs to be able to quickly shake off stun-locking effects of any kind. A good approach is to make it cost the boss something (e.g. a Legendary Action, one of its multiattacks, its recharge roll, hit points, its Lair Action that round, etc) in order to clear the stun-locking effect.
Bosses who can dramatically change the terrain or summon minions may be an exception to this guideline, but even then you probably want to give it some way to mitigate the worst of the stun-locking effects.
3. The boss needs to present a dynamic changing threat that evolves or tells a story over the course of the battle. This might be a phased boss like in Mythic Odyssey of Theros or Rime of the Frostmaiden. This might be a boss that at half hit points gets to use a special Reaction. This might be a boss whose environment changes - like my example of the shadow dragon getting crushed by sailing ship and opening up into an underground chasm below them and the party. In a sneaky alpha striking party situation, this might be a boss with some kind of illusion or double. In a situation with an artillery-focused party, maybe this is some kind of a force field that gets raised. The idea is that no matter what (barring exceptional player preparation), the fight is not a static you stand here, I stand here, and we wail on each other.