Interested in building my first PC. These are my current specs. I have questions!

Specs:
- Processor: Intel(R) Core i7-6700K CPU @ 4.00GHz 4.01 GHz
- Installed RAM: 16GB
- Graphics Card: NVIDIA GeForce GTX 1070 (8 GB)
- Storage: 466 GB SSD Samsung SSD 850 EVO 500GB, 932 GB HDD ST1000DM003-1CH162
- Running Windows 10 Home Version 22H2
- OS Build: 19045.6466

For gaming, I like the sims 4, fortnite, and fallout 4. Lately the PC has been slowing down a little and I’m also not getting the same performance I used to so I want to be proactive. Would you all recommend I replace parts or build one or buy one? I’ve never built one before but I did take this one apart once to clean the fans. I feel like I could learn. I’m also happy to answer any more questions because I might’ve missed something. Thank you!!