It isn´t a bad range of outcomes to such a nation.
Because it is horribly ahistorical.
unable to push its influence in the 30 years war
That is not completely true. Sure we got beaten, but that was due to facing the combined might of the entire Catholic side; something Sweden didn't when they entered around a decade later. Had Christian IV been better at judging when to enter it could well have been us and not Sweden who were decisive in the 30 years war.
I'm not trying to be insulting, but I thought Denmark was fighting an uphill battle throughout this entire timeline.
No, not really. As late as the Kalmar war around 1610 we were still the dominated Sweden. Sure Sweden had managed to break free earlier, but that to a good extent was due to how Christian II managed to get deposed by the Danish nobles after having cut off the heads of the Swedish ones.
What really harmed us was the German invasion of Jutland in 1623; that sacking was so thorough that a previously prosperous province got poor (and the invasion was still remembered by the common people several centuries later...). When Sweden then attacked us in 1645 (for not doing enough for the protestant side as far as I remember) Jutland still hadn't recovered and we were in trouble.
Then in 1658 we get to war again (can't remember who declared) and aren't doing too badly until a fluke where the Swedes not only decides to walk across the ice of Storebælt, but said ice actually doesn't break.
That leads to the loss of Skåneland which turns the balance; it changes from Denmark being more powerful and populous than Sweden to Sweden being much more populous and powerful---plus Sweden gains dominance of the Baltic which we had held previously. Together with Norway we can still pretty evenly match Sweden though.
The 1700s are purely prosperous times after the Great Nordic War.
And in 1801 we have the second largest fleet in Europe and the UK is genuinely scared of it; hence why they steal it when they receive (false) information about it being close to joining the French.
And yes we lost Norway inside the game period---but only by 6 years. And that situation is a really special one which you won't see in most games.
only under threat of French involvement on the Swedish side did the Danes finally retreat from their fortifications.
In fact hadn't it been for the French demanding status quo ante bellum then we probably would have reconquered part of Skåne; perhaps even most of Skåne. (Halland and Blekinge weren't feasible to reconquer in that war.)
Sweden should be able to take over skane, blekinge, and Halland
Why? A fluke should be easily reproduced? Not to mention that that Swedish stunt probably wouldn't have succeeded at any time, but in the middle of the little ice age (which was what they did it); storebælt fully frozen isn't normal at all.
Maybe put more of their development in the islands/Jutland and less in skane?
Would be ahistorical; Skåne was a very rich province and you couldn't be elected king unless you had the support of the thing in Lund.