Im adding a second SSD

Here’s the needed parts to add in the second drive (SSD in this case) iMac Intel 27" (Mid 2011) Dual Hard Drive Kit or 27” iMac SSD Data/Power Cable. And here’s the guide to put it in 安装iMac Intel 27" EMC 2429 双驱动器组件（机械硬盘或固态硬盘）. As far as which SSD drive to get I prefer the Samsung 860 EV drives they tend to be the fastest and most dependable.

As far as size, that’s tricky! You’ll need to add up your apps plus the OS to see how much that collectively takes for space, using that and knowing how big your projects are will determine the needed space. As an example I have a 2013 Mac Pro with a 2 TB SSD which serves me well working on very large images like 30 x 15 feet for one I’m working on now. I’m sure you’re not working on anything that large ;-}

My general recommendation would be either a 512 for general use or 1 TB for music, image and video work. Whats important is leaving 1/3 to 1/4 of the drive free! This unused space allows the OS to setup virtual RAM & caches as needed and with some apps a scratch space making them more effective.

Don’t forget the RAM! I do recommend getting to 16 GB at least, going larger is still iffy! Only a few apps can leverage more than 16 GB.

I would do one thing at a time making sure that is working before going to the next action on your list. As an example I would first work on the RAM making sure it works, Then I would put in the SSD drive, put things back together enough to check it out, then go back in to replace the HDD. I would recommend getting a SSHD hybrid drive like a Seagate FireCuda which is only available upto a 2 TB. Use it as your active projects drive and any library’s you use. Instead of using it as your deep storage, use an external drive for that!

As far as Bluetooth upgrade there are a few kits that where available awhile ago I haven’t found any available any more, sorry.

As far as macOS I strongly recommend you stick with Sierra! The problem with th e newer macOS’s is APFS and how it plays on older SATA based systems. Its not very good! HFS+ is still the better file system for older SATA drives (HDD & SSD).