IMHO, Linux Mint should make LMDE the default version, because it’s lighter weight and it is less dependent on other projects. Further upstream is better for these kinds of projects, if you ask me
The cautious approach for LMDE5 users: If your system is working fine and there are no especially must-have features in LMDE6, there is almost certainly no rush to upgrade. Take your time.
Make backups. Test backups. Play games. Work. Do things entirely unrelated to the distro.
You could even almost (aaalmost) completely forget about LMDE6 (but do keep an eye on the LM blog).
The Mint team haven't announced an EOL date for LMDE5 yet, but if past dates are anything to go by, it'll be at least 18 months before they pull the plug. Even then, LTS updates might still filter through from Debian proper.
[How many people will actually see this message and how many it actually applies to out of them might well include me and literally one other guy somewhere else on the planet, but if you're that one guy, breathe friend. No rush.]