The 5mm warm-white LED's are mounted as two serial loops each including a resistor of 2K2.
The whole board will draw no more than 6 mA from the rails.
Led's have a light angle of 120 degr. and or not higher than 5 mm.
When all led's are positioned with plus (anode) facing middle of PC-board, it is easy to connect a Capacitor between
plus (middle) and minus (side of board).
Capacitor of 100uF 25V will do the job. The higher the cap, the longer Led's will light after disconnection.
Between the plus and the rails, a diode should be connected.
All this will ensure that flickering is history.
With capacitors > 220 uF use a 100 ohm resistor in serial with capacitor, to reduce charge current.