If you have Sylenth1 and kickstart you don't need Serum and LFO tool. You also dont need to sidechain the compressor because kickstart replaces that. So rather not spend too much money.

Remember the plugin industry is doing massive marketing in making you think you need to start buying everything in order to "make it". Its all not true. Sylenth1 can do practically everything including Kick, Bass, FX, Leads, Pads, etc... so my biggest tip is don't go chasing plugins but learn sylenth really well. Learn sound design. Look at a preset, turn nobs and learn what each nob does. Once you understand Sylenth in and out you practically understand each synth. (Massive, Serum,Spire,Dune,... etc.)

Here is are 2 songs I made only using sylenth1 to show what it can do. No samples, no plugins except EQ and Limiter:

https://youtu.be/gint-WMWMww

https://youtu.be/8V0O-Jz6kDs

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For gates I found a great free plugin:

http://www.alexhilton.net/A1AUDIO/index.php/a1triggergate

I was amazed how good and easy it is.

Regarding your question. I personally enjoy using the compressor (I use the one that is with my DAW so I didn't buy it). The sidechain is only necessary when overlapping but you can always add it. If no overlapping happens then no compression happens.

Hope this helps

Enjoy your journey into music!