Learn English – In this phrase, should I use “cheered” or “were cheering”

In general I agree that using ago implies that the simple past would work better. In this case, however, the simple past doesn't provide the obvious and relevant connection/link between the two sentences that had planned offers.

If it were me, I would use had planned (for its link to the previous sentence) but replace ago with prior to avoid any subtle semantic imperfections. (Edit: Except you stated that the semantic intent was half a year before "now"... So this suggestion is less useful.)

With that said, neither example is particularly offensive, and I doubt that other native English speakers would have strong opinions either way if they encountered either example in real life.

Both sound correct but there's a nasty nuance :

• "The steak I had (last Sunday) was the best one I have ever eaten". "Have ever eaten" includes the present period (untill now) where you are talking ; it's still continuing actually.
• The past simple "I ever ate", means the eating is already over, ie. previously past meals.

Nonetheless, the distinction would infer/play only if you were talking with a steak (in your plate) you already start to eat... In such case, "ever eaten" would include that last steak. But so far you haven't done yet to eat it, the use of "ever ate" wouldn't allow us to understand exactly whether you're considering the meat you're eating now.