You can use of of the loaders such as TCRP, ARPL or Peter Suh's M-Shell that is based on TCRP:
https://xpenology.com/forum/topic/61839-tinycore-redpill-loader-build-support-tool-m-shell/ is the one I use.
They all now allow you to build within a web interface that makes the process pretty simple with support for many newer models. The DS3615xs has lost support since it is one of the oldest models, so the DS3622xs+ would be a great upgrade for you. But if you do not have 12 disks in your system, for cosmetic reasons only, you can choose one of the models that hold 2.4 disks.
The steps I would use is, get a new thumb drive and a spare hard drive that you can test builds with.
Download the PAT file for the model you want to use, you will need it later.
Remove you data disks from the system and put them in a safe place until the build is done.
I would keep the disks in order for simplicity.
Build say the DS920+ and see how you like it. If you are satisfied, power down the system, remove the test disk and install your data disks.
Once booted, you will get a prompt to migrate. At this point you will get a prompt to re-install the OS.
Choose manual and select the file you downloaded.
Enjoy your new NAS.