Some bits translated from Wiki:
Regarding the Soviets retaking the island in January 1942: "After the taking of the island the operative situation in the Eastern Gulf of Finland
changed to the Soviet Union's advantage. When the ice would melt, its naval units could use Suursaari as a base, which would
cause difficulties to Finland's coastal defense and would
threaten the closure of the Gulf of Finland executed by Germany and Finland."
In February 1942 the Finns retook the island the second time (first time was in December 1941) and proceeded to retake Tytärsaari as well, which the Russians tried to take back twice.
This btw reminds me of the fact that on the Gulf of Finland during winter the ice was used on several occasions to launch infantry and even armoured attacks in the case of the Russians. Currently there is no way to simulate this accurately. One might argue that land attacks on ice may be too small for DH's scope, but the Russians used forces on divisional level at least in 1940 to attack over the ice of the Bay of Viipuri.
Anyhow continuing:
"The Germans' invasion attempt
After Finland and the Soviet Union signed a truce in the beginning of September, 1944, the German forces had retreated in Estonia to the level of Narva. Finland's withdrawal of the war
opened the Soviet Navy a free passage to the Baltic Sea. So that this Soviet Navy movement
could be restricted, the Germans decided to invade Suursaari.".
This island is important! Include it!
Coastal batteries and naval mines would be nice too.