Fractal Designed cases are prices competitively. As seen here the most excellent
Define R5 is $125 compared to $150 for that
Corsair Air 540. Of course, that is here in the US. If Fractal Design cases are not available where you live, the point is moot.
It typically is not the best option.
Understand the case side panel (when installed) works to "channel" the
desired air flow, front-to-back (or bottom to top)
through the case. If you remove the side panel that flow pattern is totally disrupted and even lost.
If you are not getting sufficient cooling with the side panel on, you need to add more case fans (or replace current case fans with bigger/better fans). Alternatively, if you really just want to run with the side panel off, I would urge you to blast a desk can into the open case.
That said, conventional cooling (fans and heatsinks) can at best, only cool electronics to the ambient (room) temperature. If your computer room is not environmentally controlled, changing your computing times to the cool of night may be your only option until the seasons change again.
And by all means, you should be monitoring your temps. I use and recommend
Core Temp to monitor my CPU temps. Alternatively, I also like
Speccy (from the makers of CCleaner) which provides much more "System Information". It has a notification/system tray applet too where you can monitor the CPU temp plus GPU, System (motherboard), and drives too.