Considering Linux Lite as a lightweight distro

[stevef]

If I undesrtand it correctly Lite receives updates for 5 years, which (taking one year of overlap into account) means I´d have to do a fresh install only every 4 years. 

My first question would be: am I right in making this assumption?

There's no dates yet, but if previous patterns are followed, we can expect release 7.0 to become available in June 2024 and maybe version 8.0 in June 2026.

The fundamental release cycle is 2 years - so while 5 year support is attractive it assumes adopting early and staying with the series late which you can't do every time.  To get the same long support next time with a 4 year fresh install policy you'd have to skip series 7 and be an early adopter of series 8.