Thank you for you diagnostic very much!
1) yes, i have two fans on left and right side of the heating block, each of them is 24v, 0.08a 40mm x 40mm. And they both work when printing.
2) i adjust my belt tension last night. the results are better than before. i test my head when shutting down the machine, it has little movement, almost still.
loops on wall, it happens everywhere, i have no idea about that. maybe i'll record a video later.
stringing, i'll test my temperature tonight and come back to report.
yes! i set 0.3 nozzle size in cura. (sometimes 0.27 to adjust the flow)