Great to see different approaches and some polite, healthy debate.
Personally, I'm with Zitaliam on the team approach. I have no desire to boost two or three dragons way higher than my larger team. Sure, it will take me longer to complete the two maps and, therefore my rewards from side quests battles will be lower, but I never have a problem with events due to lacking strong dragons with particular elements. I'm happy to take the tortoise approach and let the hares streak by me! Similarly, I'm not rushing to increase their enchantment stars until I can do the whole team at the same time. I don't use sigils on any of my dragons. The first two weeks of sigil campaigns were enough for me to decide that I simply can't be bothered with it and I rarely have time (or energy) to tackle the despised dungeon. Maybe much, much later.
As for what to spend gems on. First and foremost is the portal. This means that you have enough energy to be able to hire dragons that will see you right across both maps until you reach the islands that don't allow hiring. Definitely important to do this as the rewards from those side quests increase the further that you go. If your VIP status is active, remember to use your instant wins (I keep forgetting - d'oh!) and use them on the heroic map (the one with mauve sea!) if you've reached the point where the rewards there are better than those from the campaign map. Remember that it only uses 1 instant win ticket per battle, rather than 2 portal energy on the heroic mapl.
I have just one Boss Dragon. I have three on my Windows account, which was already a year old when I started to play on Android. By then, I'd realised that they were pretty pointless.
I also concentrated on triple element dragons right from the get go. This meant that my three free enchanted dragons were triple element. In fact I managed to hold out long enough so that they were also light and shadow dragons

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I have upgraded my legendary habitats so as to get more space for my dragons.
I've also upgraded my nest. For a long time, it was at three and then four. Only quite recently did I have enough expendable gems to get the fifth nest and now I'm not sure how I coped without it. That said, unless you're going to buy the second den, you really won't need so many nests.
I very rarely spend gems on the whale mart. I'm not a gambler, so I'm not even tempted by things like the ancient tickets. And I certainly don't want sigils.
I only awaken the elements of Legendary, Divine and Ancient dragons if I think they are going to join my team.
I've upgraded my dens in order to be able to breed extra dragons, although many of them I can't yet breed because I haven't enchanted the parents.
Nearly forgot - I also upgraded the Fruit Tree (with my solo Boss dragon) to level 5 (so far).
Other than that, I save my gems for rainy days - ie something like completing the necessary doubles to fight the battles for the Ox. Currently, I'm holding out, still hoping for some more lucky throws!