You seem to have confused some of the events from your research so let's start by setting the record straight (elaborating on the useful insights from @Armadillo's answer):
MER (Spirit & Opportunity) Launch (first link):
Launched on a Delta II rocket that required a third stage Payload Assist Module (PAM-D), a Star 48 solid rocket motor (basically). The third stage PAM-D was spin stabilized at 30 to 110 rpm (Delta II Payload Planner's Guide, ULA 2006) before firing and yo-yo despun to "12 +/- 2 rpm" before separation (Portock, B. et al., "Mars Exploration Rover cruise orbit determination," 2004). During cruise the MER spacecraft were spin stabilized at 2 rpm and used thrusters to reduce the 12 rpm spin to 2 rpm (Portock, B. et al., "Mars Exploration Rover cruise orbit determination," 2004).
Mars 2020 (Perseverance) EDL (second link):
Launched on an Atlas V 551 and did not require a third 'kick' stage. Before separation the launch vehicle imparted a spin* to the spacecraft which it later corrected, with thrusters, to a speed of 2 rpm (Mizukami, M. et al. "Mars Science Laboratory Cruise Propulsion Maneuvering Operations," 2013). Prior to entry, the linked animation states that "the spacecraft uses thrusters to stop its 2 RPM spin" i.e., it does not use the balance masses to de-spin. The next phase of the animation states, if viewed in full screen, the two balance masses are 70 kg each. Later in EDL, during the SUFR (straighten up and fly right) maneuver, 6 additional balance masses are ejected, massing 25 kg each.
*note that though it looks like the spacecraft begins to spin after separation, that is in reality the second stage de-spinning, like with MSL (Curiosity)
For spin stabilization & yo-yo de-spinning there are a few question & answers here that may help:
Calculating the weight of the balance masses:
M2020/MSL are unique missions where the location of the centre of gravity/mass (C.G.) of the aeroshell/entry vehicle changed multiple times during flight. It's easy to understand needing a C.G. located on the axis of revolution of the radially symmetric aeroshell/entry vehicle while spinning during cruise from Earth to Mars; everything is mounted, and therefore should ideally spin, axially.
From Schoenenberger, M. et al. "Assessment of the Reconstructed Aerodynamics of the Mars Science Laboratory Entry Vehicle," 2013 (NTRS ID: 20130010088), where the axis of revolution is the vertical dashed line cutting through the middle:
During entry, the aeroshell/entry vehicle generates lift by flying at an angle of attack. This angle of attack is generated by offsetting the C.G. from the axis of revolution, the x-axis in this figure (again from Schoenenberger, M. et al.):
(you can also see the greenish/brownish regions where the balance masses are ejected from in the image above; 2 in the +z direction, 6 in the -z direction)
Using the design parameters shown, it is easy to determine a good estimate of the required mass for balance during cruise by performing a moment arm balance:
$$z_{cg}/d=0=\frac{(3152.5 kg \cdot (4.5 \cdot -0.0215))+(m_{balance} \cdot 2.35)}{4.5 \cdot (m_{balance}+3152.5 kg)}, \to m_{balance} = 129.8 kg$$
where $2.35$ meters is an estimated z-axis location of the cruise balance masses. This is very close to the total 140 kg (2 x 70 kg) actually used.
Also shown in 'Figure 6' is the SUFR (straighten up and fly right) maneuver which returns the vehicle back to a zero angle of attack attitude (ideal for parachute deployment) by returning the C.G. back to the aeroshell's/entry vehicle's axis of revolution. We can use the same method to calculate the required mass for this maneuver:
$$z_{cg}/d=-0.0215=\frac{(3152.5 kg - m_{balance}) \cdot (4.5 \cdot 0)+m_{balance} \cdot -2.2}{4.5 \cdot 3152.5 kg}, \to m_{balance} = 138.6 kg$$
where $-2.2$ meters is an estimated z-axis location of the SUFR balance masses. This is very close to the total 150 kg (6 x 25 kg) actually used.
