Your results are likely very similar to this:
This was measured by
Archimago and is a turntable with the same 'motor'.
The result is rather good for a Direct drive and or servo controlled belt drive.
It is hard to say IF and how it can be improved.
The Technics (and other direct drives) work on the principle of a multi-pole (4 x 3 phase) motor.
There is also a speed servo 'ring' in the middle of the platter and position pick-up(s) on the motor assembly
The TT PCB has a quartz X'tal AND a free running oscillator (for pitch control)
That 'reference' frequency (which is divided a couple of times depending on the selected speed) is compared to the frequency that is coming from the pick-up coils in the motor.
When the desired speed is reached the PLL (Phase Locked Loop) in the motor circuit will try to keep the reference frequency and the signal coming from the FG coil in the exact same frequency AND phase.
This could be lowered perhaps by increasing the weight of the platter (heavier mat or weight).
BUT if 'overdone' the platter speed may become more unstable as the motor is designed to handle a certain weight acting as a flywheel to obtain a smooth rotation.
The spindle bearer is also made for a certain weight. Furthermore the platter itself is 'balanced' in a similar way as car wheels are balanced.
So... sticking on materials to make it heavier MAY make things worse.
The 'floating' frequency (wow) may be caused by the way the PLL is constructed or could be a construction tolerance as it shows a wobble during a rotation.
That PLL circuit compares the two frequencies and when they deviate a small 'error voltage' is generated. This is 'demodulated' with a low-pass filter which makes the circuit 'sluggish' and always a bit 'too late' with its speed correction.
This means: when the platter is going slightly too fast after a short while the circuit sees a voltage rising and the motor 'power' is eased off just a tad slowing it down.
When the speed becomes too low the same thing happens again but in the other direction... the circuit 'sees' the platter speed dropped too much and the motor 'power' is increased ever so slightly again making it spin-up a bit faster again.
This goes on and on and this is the large 'speed variation' you see.
This is more or less sinusoidal because of the construction of the 'PLL filter'.
Little can be done about this as it is a mechanical/electrical/electronic combination. The PLL filter could be made 'faster' so that it reacts faster to a changed speed but that would make the 'wow' also a lot faster (and more audible) and may even give stability problems. Perhaps in combination with a heavier plateau it may help a bit.
The 'gain' of the PLL motor control could be 'increased' so it will react sooner to speed deviations. This, however, may mean on start-up the motor may 'hunt' a while as the 'range' the PLL circuit has was made smaller (due to increased gain) and may take a long time to find its intended speed as it over- and undershoots too much trying to find the proper speed.
After all the engineers who designed this lovely, and widely used Technics motor, knew what they were doing and engineered it so it could start and stop fast and remain stable. A trade off for various aspects of the TT. A heavier plateau would have meant different bearing, more metal and a different motor control. It would not start and stop fast (which DJ's want).
Hooking up another power supply isn't likely to improve the platter speed/stability as that is determined by the PLL, motor current and platter.
It would be interesting to see IF numbers and graphs improve with a better power supply but I reckon it won't.
Be sure to keep the phone (with the app for measurement) very still (in a cradle ?) as this influences the measurement.
A tech that can add 'weight' to the platter and has the ability to tweak the PLL and measure the speed may get some improvements.
The 'speed hunting' may become somewhat less and slower with the right tweaks.
It puts digital audio in a different light as the 'wow' and 'flutter' (jitter) of even crappy equipment is >1000x smaller than the best TT's out there as far as 'wow' and 'flutter' is concerned BUT due to the weight of the platter there won't be any flutter at higher frequencies which MAY be there in a jittery signal.
Very likely you won't even be able to hear the 'varying' pitch of the music so it isn't that bad.
Won't even mention what happens on the cutting table (that speed isn't constant either) nor do we know how 'good' the spinner was on which the test disc was cut.