Yea, i think you get the idea, just giving different view point.

bit of math/comparasion

pro and con - right now you might struggle to complete a event, but generally most of them you can complete with the different maps, just do the easier maps. The harder amps might be more efficent, and sometimes drop ampys, but that is more like icing on cake. should not worry about that.
if you hold off on spending FG, then you get more value for Gem, ie spend gem now for 100-150 stam or spend it later for 300 stam. Having said that just got 300 stam myself, and i find i do not need to buy stam cause i find i have enough to do events.
Even doing Manyu missions days twice a week i still not short of stam for events, more would be better but not a issue for me.

Stam is really short supply at lower levels, but if you can make it to higher levels, it becomes a non-issue.
One trick i find extreamly helpful is timing my levelling up to get that boost in Stam at the right time, really makes things alot easier, i do 12-5 map 3-4 times to get the next level instead of running 2 even maps, and get Stam refill and the 3 extra gathers(ie more stam), nothing new BUT really helpful, do it all the time, only time i stuffed up was trying to rush and maintenance caught me with full stam, lucky it was only 1 hour.
If you can NOT do 12-5, no mater, same principle, do normal mission that you can manage with good exp, get that new level and then back to event missions.

Even if you do not like the current knights now available, saving up FG is better because you "might" want to roll on a Gacha later that does have a knight you like.
For me it was Fir, save up 120-130 gem since I started playing, tried two 10+1 roll, got NOTHING, not even a 5* knight. Sacrificed expanding inventory to save thoes gems, THEN i managed to get another 50, and did one more roll and Got Fir. Yes i was lucky, but if i did not save thoes Gems, then i would not have Her.
So many times i considered spending toes gem (the bride Gacha was tempting) but i held back, and am thankful i did.

So point is - Save for Rainy day.

This all implies you planing to play long term, if you thinking of not playing alot in future, spend those Gems