Motors will require a high torque output without too high RPM. 20-50RPM should do it (max speed between 0.1m/s - 0.26m/s). Expected weight will be somewhere between 10-15kg, with a maximum incline climbing capability of 10º in dirt. For a 15kg robot, this means that the necessary force output has to be:
$$ F_{friction} = \mu \cdot W = 0.6 \cdot 15kg \cdot 9.8 = 86.95N \\ F_{incline} = W \cdot sin(\theta) = 15kg \cdot 9.6 \cdot sin(10º) = 25.55N \\ F_{total} = 86.95 + 25.55 = 112.5 N $$
Which means that, each of the 6 motors will have to apply a minimum torque of:
$$ \tau_{motor} = \frac{112.5N \cdot 0.1}{6} = 1.875Nm $$
For increased acceleration, each motor should apply between 2x and 5x that torque, so between 3.75Nm and 9.375Nm.
For that, the motor should have a power between 7W and 20W.
The chosen motors are the following (37GB555):
https://es.aliexpress.com/item/1005003181068931.html?gatewayAdapt=glo2esp
The motors will be driven with a H bridge DC motor controller (ZK-5AD):
https://es.aliexpress.com/item/1005006308936405.html?spm=a2g0o.order_list.order_list_main.11.435118021PsgLS&gatewayAdapt=glo2esp#nav-specification
The motor is smaller (20mm in diameter), so the dimensions of the wheel will have to be reduced.
The ZK-5AD allows for an input of maximum 14V and outputs up to 5A per channel. It allows independent control of 2 motors.
Motor D0/2 or D1/3 in opposite states (High/Low) allows for changing the direction of motor 1/2. Using a PWM in either of the input channels allows speed control in open loop of the motors. The duty cycle of the PWM signal controls the voltage that the controller lets flow into the motor. A 100% duty cycle allows for full battery voltage into the motor.
This kind of controller only allows speed control, no torque control. Since it is a very simple control schema, the low-level speed controller for the motors will be quite simple. Open-loop speed control will be applied (assuming 100% duty cycle will make the motor spin at 30rpm).
Low-level control of wheeled robots: https://sci-hub.hkvisa.net/https://ieeexplore.ieee.org/document/7926529
High-level path following control: https://www.hindawi.com/journals/tswj/2014/793526/
Exomars controller: https://robotics.estec.esa.int/ASTRA/Astra2011/Papers/02A/FCXNL-11A06-2141876-5-2141876Lee_Paper_WithCorrectedTypo.pdf
Motor control of wheeled robots: https://www-sop.inria.fr/members/Pascal.Morin/PapersDR2/chap_08.pdf