Depending on your case your fan slots may vary, however, they should always be like this:
Blue is intake (cold air)
Red is Exhaust(hot air)
Circle/cross thingy is for side fan, since some cases have their position different, some very low, some middle some high
If your side vent is HIGHER than your GPU make it exhaust
is your side vent AT/BELOW the GPU make it intake
Never make your "back" /"top back" intake, since you just blow the hot air from the gpu/cpu/chipset right back in the case with no solid means of venting
Best air flow comes from- lower front to top back, thus letting cool air stream over hot components while effeciently drawing the hot air air that rises out at the same time,
Low exhaust is pointless since heat rises,
For a positive air pressure case flow, prioritize fans as such:
1 front intake, as low as possible, meaning if you have 2 or more front mounts start with the 1 near bottom of the front panel.
1 back exhaust
1 side vent preferable intake(in this scenario if your side vent is mounted higher than gpu dont use it, as we want it for intake, instead add another fron intake)
That is the best standard 3fan setup, this should be your starting point
if you later want to add fans, remember for positive pressure= more intake than exhaust, so always apply them in that order
Now for the rest: dont worry if your mobo only has 2 fan headers, u can safely connect 2 fans to each header via y splitter, adding more than 2 fans per header depends on fan type and their power usage
and even so, since you are running your fans at full speed those you choose to run at full speed you might as well just connect directly to a molex, gives same result
(regarding RPM i did warn you that fans could be loud specially at higher RPMs
😉 )
Heat spikes can be common in certain scenarios, since you atm might not have the optimal case cooling that could be one,
another could be an application that intermittently uses the gpu however brief that cause the spikes, some background apps have actually caused such in the past, as long as it doesnt blast it up in high temps shouldnt be worrisome, but can always check your running apps if you do.
Your case arent that bad for cooling, it has all the essential fan mounting places, and in a solid amount
unobstructed airflow from lower front to top back, if you can remove any cage and bay you dont use/need