Why don't you sync with the chain on your PC? The processing power of Pi restricts the speed of sync and this might be also a workaround for your problem.
I want to test all the process in a weak environment to tune it
Some thoughts:
The question becomes then how weak an environment do you want?
You are using an external drive, but you can get 512GB good fast name brand SD cards for ~$125 USD (Samsung / Sandisk / PNY) you can get off brand ones that are still OK for less. But their performance sucks.
You are using an external drive, which is good. BUT are you using a 5400 RPM or a 7200 RPM or a SSD?
Setting up the swap space and configuring node are going to be ALMOST the same process, BUT not 100% the same. You are going to get different answers in what is going to get you the best way to do it depending on what else you have. With a good fast SD & SSD you can use more swap with a smaller performance hit to other I/O operations then with a generic slow SD and 5400 RPM drive.
And don't get me started on USB -> SATA adapter quality & speed.
-Dave