Most lightweight Ubuntu-based linux distro 2021

I'm a Ubuntu user, but my computer is getting older so I cannot use Ubuntu 20.04 LTS. I'm used to use Lubuntu 18.04 LTS and that works, but the support time runs out this year. So I need to replease current linux OS for a better one. The current linux OS I'm using is AntiX Linux. It's super fast, but It's quite weird because everything is old here. Old software and AntiX is based on Debian 10. I don't have so much problem with AntiX Linux, except when I try to install software. These softwares are so old, that I dosen't support some files or functions e.g GNU Octave. So I need a Linux system that are based on Ubuntu, but take less memory than Lubuntu 18.04 LTS or as much as 18.04 LTS. The support time is important too. What is the best lightweight Ubuntu based linux distro 2021 that follow this requriments: - No more RAM usage than 256 Mb - Pentium 4 or less - Support time to 2025 or more - Based on Ubuntu - 64-bit