My $.02
I made a CarbonCopyClone *bootable* backup, based on recommendations given in this forum. This allows me to boot my system into an alternate disk, in case your primary boot disk fails.
In your case, "failure" can also be defined as "too little disk space left".
You may want to get a self-powered external disk that is larger than (perhaps 2x your primary disk) to act as a "bootable" alternate in case your primary ever fills up and fails to boot.
That will give you *something* bootable to clean up your primary disk.
You may also consider upgradng your internal drive as well, but implementing an external bootable clone is faster and solves your IMMEDIATE risk of your system disk deciding not to boot.