If you get all of the rares, you don't need 3500 points for the summon rush.  The 3500 points is only needed if you want to also get the epic.   The rare in Summon Rush will be about 1750 points I expect, which is easily achieved with your shard count.

And yes,  you are correct in that there are only 5 rares available up to now:

• Dungeon Divers 1
• Dragon Tournament
• Champion Chase x 2
• Classic Arena 1

Since you have gone all in for the Champion Chase and gotten 2 rares from it, I would target just the first rare in Summon Rush and complete the fusion.  Don't spend all of your shards to get to 3500 points if you don't need the epic (which isn't great in my opinion).  Save the shards for the next event

Edit: Just to summarise, in total there are 16 rares to collect, and 1 epic.  But you only need 4 epics to fuse to the legendary.  If you collect the epic from Summon Rush, then you only need 12 rares to fuse the other 3 epics.  If you collect all 16 rares, then you will get all 4 epics from them, and therefore don't need the 3500 point epic in Summon Rush

Edit2: Keep this image open in a tab as you progress each day to make sure you are on top of each of the rares: https://plarium-multimedia.s3.amazonaws.com/5E5C56F2-2BFA-4EB2-A530-23125CAEC2A9