MacBook Pro mid 2012 speed issues

Hi, I am currently using a MacBook Pro Mid 2012. Here are the full specs..

OS High Sierra 10.12.1

MacBook Pro (13-inch, Mid 2012)

2.5 GHz Intel Core i5

8 GB 1600 MHz DDR3 (has been upgraded from the standard 4GB)

Intel HD Graphics 4000 1536

500GB SATA Disk

About 6 months ago I noticed my laptop starting to run slower. It has now got to the point where it is practically unusable. I recently upgraded the RAM to 8GB hoping this might help however I have noticed no improvement what-so-ever. I have tried everything to get my laptop running at a usable speed again, updated OS, erased HDD, reinstalled OS, deleted Caches, reset SMC, reset NVRAM, etc. I have considered installing an older OS but I use my laptop mainly for music production and having the latest version of Logic Pro is very helpful for me. I have also considered installing an SSD (does anyone has a suggestion for a good one that may help?) however I have read that in some cases they have not improved performance (also these can be expensive). I am a student and I haven't got much money and need my laptop for work so this is extremely frustrating for me. I have contacted apple support but none of their advice has helped. I bought the MacBook just over 2 years ago refurbished from Apple but unfortunately the warranty has run out so I am afraid to take it to a Mac store as I know their repair prices can be extortionate. If anyone has any suggestions I would love to hear them. This whole experience has made me feel completely disillusioned with Apple as a company. Thanks for any help you can offer.