This might not be the best section since not really metrology related but since nobody else is answering I'll take a stab at it for what we do in the lab (You might ask in the ham radio section and that might get you a better-quality answer from those guys):
1) Your brass posts are probably fine for simple grounds.
2) Your cheap PC case is splattering RF everywhere. Probably should address that if you can. Good cases don't cost too much that are rated for better EMI shielding. Shield your SDR also. Also any switching power supplies (most everything) are noise generators, so move those away. Most SDR's I've seen are not shielded very well themselves, if at all.
Also watch out for fluorescent or LED lights - those can spew RF absolutely everywhere.
3) You might try a star ground, not a looped-around-the-shack shared ground. i.e. Pick ONE good ground point and run separate ground leads to that single point - best to use a ground rod nearby if possible. We like braided ground lead, not solid wire, for a little better performance - but use whatever works. The really good stuff is the copper braided downlead for very broadband lightning protection, if you can get that. But experiment with whatever you have, and try different setups to see which is best.
4) Deoxit goes on, work in to clean, then wipe off excess. Be practical about it - you don't need a lot. If you leave a lot of excess on it can get gooey and attracts dust.
5) Use a good -solid and tight- crimp connection, and maybe solder afterwards if it makes you feel better - a good, tight crimp is usually best. Make sure you have the right crimper for your wire size and connector. Or use a GOOD screw-clamp connector for your ground leads. Nice and snug but don't over-do it.
If it helps, there's an article in Circuit Cellar a couple months ago that addresses trying to run an SDR from a PC. It turns out the better result was running the SDR remotely using a Raspberry-Pi mini PC: You put the RaspPi and SDR up in the attic (or wherever) as far away from all the RF noise as much as possible, and control the SDR with the RaspPi using WiFi or Ethernet cable from your PC to Rasp-Pi. The Rasp-Pi is small and can be shielded in a small metal box, and makes a lot less noise than your desktop PC. In other words get the SDR as far away as possible from that noisy PC. You'll have to look up the article but its from a month or two ago - fairly recent.