In World War 2 the Soviet Union suffered the most military casualties by far. How much of this is due to Soviet strategies and decisions?

When you look at apples-to-apples figures, keep in mind the context in which those figures were generated. The simple fact about the Soviet front that must be taken into account is that the Soviet Union was big. Very big. Soviet strategy relied on the Soviet Union being big. Soviet casualties far outstripped, say, American casualties, because Soviet involvement in land warfare against the Wehrmacht was on a whole other order of magnitude from that of the Western Allies.

Also, you've been misinformed about Soviet strategies and doctrines. First off, the Soviets did use extremely conservative doctrines. They began the war with doctrine that wouldn't be out of place in early inter-war exercises (purging the most creative elements of your officer corps in favor of the less threatening officers tends to cause your army to adopt less radical doctrine). As a result, you still saw a reliance on the shock of concentrated and coordinated infantry, heavily supported by artillery, with tanks distributed piecemeal throughout as support.

Their enemy had inherited a Prussian military tradition that had largely perfected maneuver warfare against much larger, less maneuverable foes. In a way, the Wehrmacht of 1941 was the single worst enemy for the Soviets to fight - Soviet massed infantry armies were slow to respond and easy for the Wehrmacht to encircle and reduce. The largely unprepared Soviet officer corps was undone and overwhelmed by the sheer scope and audacity of German operational maneuvers.

'42 and '43 saw a major reform in the Red Army even as the Wehrmacht overextended itself and literally froze in place - the disgraced Tukhachevsky's ideal of "Deep Battle," "Attack in Echelon," and "Consecutive Operations" came back into vogue, and the likes of Zhukov took the reins of the Red Army with a commanding knowledge of how to play to Soviet strengths and German weaknesses. Namely: The Soviets had so much more to work with than the Germans. Famed historian David Glantz (probably your best Western resource for research on the Soviet front in WWII) identified these major Soviet operations that occurred or were planned to occur in 1943:

Defense of Kursk
Reduction of the Orel salient
Assault on Belgorod and Kharkov
Thrust against the Germans arrayed in front of Smolensk and Roslavl
Offensive in the Donbas
A drive on the Dnepr River
Assault at Bryansk
Front-sized operations in the Caucasus area

The Wehrmacht's reserves could not keep up with this withering operational tempo. Whereas each of these offenses was executed by relatively fresh or rested Soviet troops, the same German divisions were shuffled across the Eastern front in an effort to stem the tide of Soviet divisions, armies, and fronts. The Wehrmacht's high strategic mobility and infallibly aggressive counterattacks managed to slow Soviet advances, but a simple lack of men and materiel meant that it was very rare for the Germans to ever stem the oncoming strategic tide of Soviet divisions.

What this meant, however, was that two extremely aggressive mass armies were clashing over geography so vast that it required millions of men in order to form a cohesive front. Out of sheer volume of men, tanks and artillery, casualties soared on both sides, and would have done so regardless of what specific strategies one side employed as opposed to the other.

Where the misconception of callous "human wave" attacks may come from is a pattern that failed Soviet offenses tended to fall into. Soviet offenses were primarily organized in multiple echelons of multiple divisions - the first echelon broke through enemy lines and engaged the enemy's local reserves. The second echelon broke those local reserves and engaged the enemy's strategic reserves. The third echelon broke the strategic reserves and they and follow-on troops ran roughshod over the enemy rear. In theory, the idea was masterful, and when it worked, it worked extremely well - the most famous example is Soviet forces smashing through the Romanian 3rd and 4th Armies at Stalingrad.

Reality did not always conform. To quote Robert Citino:

What happened, however, if that first echelon failed to crack through? A second one was behind it, waiting, and the commander's response was almost always the same: insert the second echelon to complete the breakthrough that the first had begun [...] launching a frontal assault against an enemy position that was by now fully alert and had already defeated a first attack. Here we have the German observation of a Soviet tendency toward "senseless, wild hammering" in battle [...] long after any possibilities for success had vanished.

Keep in mind, of course, that German generals 1) were in constant shock throughout the war at just how abysmally their intelligence apparatus underestimated Soviet numbers, and 2) constantly disparaged the Soviets as relying on numbers to win the day. The common post-war narrative from German generals was something akin to, "We would've won, but Hitler was an idiot, and the Soviets were an endless, faceless horde of slavering communists who didn't care about the lives of their men." This highly biased and unrealistic view of Soviet strategy heavily influenced NATO doctrine and Western understandings of Soviet thought, especially the portrayal of the Red Army in Western media.

Let me emphasize again - all of this Soviet thought represents fairly conservative doctrine. It seems cold and ruthless from a Western point of view that speaks glowingly of the art and genius of war, but Soviet doctrine largely dispensed with this adulation of artistry and viewed war as a military science for which there were optimal and sub-optimal answers. Several Soviet generals advocated for more aggressive, risky strategies during the course of the war, but were invariably overruled by others who thought (rather correctly) that a slow, but relentless series of strategic offenses across the entire theater of war would reliably break the Wehrmacht at significantly less risk to Soviet divisions, armies and fronts.

TL;DR: Soviet casualties were highest of all major players because of the staggering scale of the Eastern Front combined with a distinct lack of preparation for war in '41. These casualties were not due to Soviet commanders carelessly tossing their men aside in a "human wave" mentality. Soviet doctrine was far more conservative and cautious than one might think.