You are right, and FDev suggest to focus on Alert systems and kill Othrus to avoid the switch from Alert to Invaded.I put the answer above like this because I thought it is still the most effective activity to regain control systems.. maybe I am wrong with that (?).
Two "fronts" strategy, avoid Thargoid invade new systems, regain control of invaded ones.
That happened for Taranis, first to come, first target of coodinated efforts... Leigong next.
I think "most effective" need to be balanced with "most fun to play", redo same stuff hundred times become boring.
If alot players have fun while doing any AX activities, will be effective in the "big scheme", and seem there is alot of new and returning players, at least from forum posts.