A lot of this is possible with publicly available APIs, but each element may have its own unique limitations. For example:
ECG - currently, this is only performed on a recent (series 4 or higher) Apple Watch in the ECG app and is then published to HealthKit which you can extract with permission.
SpO2 - similar to ECG, but also randomly sampled when the user appears to be resting - also extracted through HealthKit
accelerometer - you have a few options - if you want to fire up the sensors for a short period of time, you can use CoreMotion in a Watch app which would require some user intervention. If you qualify for a research study, you may be able to use SensorKit which can extract 24/7 accelerometer data from the watch and/or phone, but only have access to the data after 24 hours.
HRV - heart rate variability is another intriguing measurement that may have to do with sleep/stress that works a lot like SpO2 in that you can force a measurement and heartbeat series via a Mindfulness session on the watch app and the os will occasionally passively measure this throughout the day when the user is at rest.