@alexmilman What then would you say is the most we can plausibly do to curb the destruction of the war to come, and what’s the best way of doing it?
I'm afraid that you are asking too much of me (of course, I'm flattered

). How can one predict with any reliability something in a situation so complex?
We can probably guess about some ...er... "critical points" which may (or may not) make the whole thing shorter if everything is going differently:
1. The "Bohemian period" goes well for the Protestant side, which would most probably require combination of (a) serious direct Ottoman help and (b) much more efficient Transilvanian involvement and (c) much more talented (and popular) Protestant leadership and (d) by whatever reasons an absence of the Spanish help. As an option, the Dutch are breaking the peace and (with a timely help of the English and Scottish volunteers) start an early offensive against the Flanders. One way or another, Spain does not have troops and money to spare to help Austrian Hapsburgs. Bohemia is lost and nothing is to be done about it except for a possible reshuffling of the electorate.
2. After Bohemian and Palatinate Wars Ferdinand is an embodiment of a religious tolerance and political flexibility (and I can sell you Brooklyn Bridge really cheap ....). The whole thing can be treated more or less as an unsuccessful revolt against a legitimate sovereign (nobody can approve such a thing) and the things are back to the Augsburg Peace. Calvinists are unhappy but that's their problem. Ideally, Ferdinand (after being hit on a head with a really big meteorite) expands existing treaties to the Calvinists in exchange to everybody's agreement regarding future succession.
3. After Denmark is beaten, Ferdinand (again, after suffering a severe mental shock, which in his case could produce only positive results) leaves Wallenstein in his position. With the 100K soldiers stationed over a big part of the HRE and a person who (one of all clowns involved) seemingly has some "imperial" thinking, the electors are getting some ...er... "spiritual experience" (or whatever) when it comes to the issue of succession. Posting more troops are posted on their territories (and levying more taxes to support them has to be paid) proves to be quite educational and even the dumbest among the princes are capable of figuring out for whom they should vote. Probably, with 100K still being efficiently handled Swedish invasion may not happened (IIRC, after W's dismissal Tilly was in charge and to a great degree existing army disappeared all the way to people going to the Swedish service when GA landed). Or, if it does, GA may be contained in the Northern Germany.
4.
France has army earlier. Louis XIII inherits an army from his father (who in OTL disbanded most of what was there) and keeps building it up even prior to Richelieu's accession. In OTL, he inherited an army of 14,000 (in theory the number could increase up to 50K) with up to 50 cannons. Louis XIII increased it up to 28,000 but the 1st massive recruitment happened only in 1636 when there was a danger of the Spanish invasion. In 1638 French army had 36 national regiments and 25 foreign regiments (each up to 2,000 strong) and in 1640 Richelieu was bragging about having 150,000 infantry and 30,000 cavalry (presumably, the real numbers were 2 times smaller but even with such an allowance it was an impressive force). In 1667 (Louis XIV, War of the Devolution) France had 60 infantry regiments (including 14 foreign) and 34 cavalry regiments (including 8 foreign), in other words,
real 150,000 (with artillery, etc.) of a standing army. It seems that by the beginning of the 30YW French army had on a permanent base 6 «Les Vieux Corps» (old regiments) and 6 «Les Petis Vieux» (junior old regiments) and the Guards. Now, if the buildup program starts at the beginning of his reign (during regency of his mother or immediately after it was over) he may have an army of 80 - 100K ready for a war. If it is led with a reasonable competence and there is a backing from the French supporters in the HRE, then the Hapsburgs can be defeated reasonably soon. OTOH, I have a sneaking suspicion that in this scenario France would concentrate its military effort against Spain in the Flanders with a possible extension to the Rhine, aka, its involvement against the Austrian Hapsburgs would not necessarily be extensive. Even then, if Spain is out of war earlier, then a general peace can be negotiated well ahead of the OTL schedule.