I managed to collect every decoration from this year without buying any loot boxes so I would just like to share how I did it. Even thou I spent some ~1500 shards on previous collection and crafted every decoration from this one, I still had 4,5k shards left at the end of the day. Here is a quick pic of full collection:
Basic tip is to complete every mission and get every small box possible.
But primary strategy is to just spam random tier V decorations in decorider from the moment you get 360 shards (from day 1) and never craft any tier IV and lower or tier V with 720 cost and 1080 (until the last day). Since breaking tier V collection gives you back 250 shards, every random one basically costs 110 shards, and it gives you the best ration of chance/cost. Do this until you have some 20 tier V decorations missing, build next few with 1080 option until you have 18 left, and the last 18 with "antiduplicator". Then craft missing tier IV and III decorations (I had 2 tier IV and 1 tier III left at the end of the event without crafting any before). And you are (hopefully) done! I have a feeling that chance of completing collection f2p way, earning every box, is very high, if you use this method 😉