If you love classic 2D RTS games like Warcraft 2 and other similar titles, Wyrmsun is one you should take a look at because not only is it free and open source but it's seeing major upgrades too.
Read the full article here: https://www.gamingonlinux.com/2020/12/free-and-open-source-2d-rts-wyrmsun-40-is-out-now