You do seem to be at a crossroads between Cost and Performance and if money was no object, I would ditch the Synology nas and replace with a NAS/SAN with SSD drives unit that takes Fibre Channel connectivity.
On a cost effective/ safe choice, option 2 as @Grisu says seems to be the safer option, it will take more time but seems to easier in practice. breaking mirrors and setting up again always worries me (need a good backup you will) 
And treat yourself to a UPS whilst the wallet is out, if you get a power failure whilst the new disc is in RAID rebuild mode, you could have issues
Black Friday /Cyber Monday soon for those deals maybe ;)
In a completely sane world, madness is the only freedom (J.G.Ballard).
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