Bro - take your time. If you're underlvl, you can ask some of the forum dudes and dudettes for their friend code, so you can have a strong helper squad to help

For Xmas event, you can just grind for cards on the lower lvl missions

I remember as a new player, I didn't finish the first July mission or Geranium's mission. That was okay. It took a month for me to atually clear events easily. This game is not that hard compared to other Nutaku games that requires a ton of grinding. I blew Flower Gems recklessly b/c I never found this site nor any other sites' beginner guide. Regret not hoarding Flower Gems.

It really depends. I occasionally use FGs for Stamina refills for the Manyu daily, but then, my maximum stamina is 370, which means I can do the 80-stamina map four times for one Flower Gem. Shortly after I passed 320 maximum stamina, we got the "bonus chest" on the daily maps, and that was just enough for me to push "not worth it" over to "totally worth it".
But, I personally wouldn't do it if:
-I had less than 320 Max Stamina (so couldn't do 4 runs on 1 FG)
-The "bonus chest" didn't happen (bug or not, it made using FGs for stamina worth it for me)
-I couldn't beat the 80 Stamina maps (Less or no chance at lvl 100 Manyus/Dragons make the lower maps useless to me)

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

It all depends on the player.

Me for example, I do not like being behind when playing MMOs. It's why I often avoid jumping into older games and dislike having to play catch up, an exception is when there are fresh new servers added but that doesn't apply to these kind of games. Though thankfully for this game, there is no PvP systems and no in-game interactions, so I don't have to worry about competing with others.

Also I do not want to be spending an hour just to burn through my stamina farming low level stuff when I can just farm higher level stages in less than a quarter of that time.

So if I ever need to spend a FG on stamina, I do it. Though I only do it for dragon farming as I can't be bothered to wait an entire week.

Gem is rare so you shouldn't spend it on stamina refill. Since you are tired of doing gacha you can use fg to buy manyus and ampys in shop, it's also an efficient way of using gems (Dont take this opinion serious ) . Otherwise, doing map 12-5 or whatever map in stage 12th campaign to get the bonus stage, i have done it before but mostly for leveling up. Also on Manyus event day, log in every 4 to 5 hours to maximize gathering.

cause everytime i pull 10+1 it always giff me bad draws

dunno why i'm absurd at gacha things

If you're unlucky at gacha, kepp your flower gems for tichek gacha. Draw a 10+1 and get a gold ticket. At least you will get 1 5* girl (unless if you are VERY unlucky and get a 5* you already have).