found at https://support.google.com/chromecast/thread/874492/is-it-possible-to-set-default-volume-on-chromecast?hl=en by user Eric Repec — I don't believe this update actually works as advertised: https://support.google.com/chromecast/thread/62025453/recent-improvements-to-your-cast-experience?hl=en

chromecast-volume-normalizer.py

# —— by default, chromecast volume is not saved across reboots — dumb.
#
# UNTESTED
#
# A python 3 script to discover all the chromecast devices and save the volume.
# Then each time you run it the script will look for any CC that has a volume of 1.0 (100%) and reset it back to the saved value.
# Safe harbor:  No warranty, no guarantees of any kind.
# Licenses of supporting modules are their own licenses.
# This is only an example of what you can do not real code.
#
#  pip3 install pychromecast
------------------------------------------Start of code----------------------------------------------------

import time
import datetime
import pychromecast

#object to hold all needed data about a chromcast device
class castDevice:
    friendly_name="unknown"
    volume_level=0.3

#array of caseDevices
castDevices=[]
f=open("pycastvolume.csv","r")
listCastDevices = f.readlines()
print(datetime.datetime.now(),"Reading ",len(listCastDevices)," devices from file.")
for line in listCastDevices:
    line=line[:-1]
    items=line.split(",")
    cd = castDevice()
    if len(items)==2:
        cd.friendly_name=items[0]
        cd.volume_level=float(items[1])
        castDevices.append(cd)

# chromecasts = pychromecast.get_chromecasts()
stop = False

while not stop:
    print(datetime.datetime.now(),"---------Loop-----------")
    chromecasts = pychromecast.get_chromecasts()
    for cc in chromecasts:
        # wait for the current chromecast data
        cc.wait(10)
        strFriendlyName = cc.device.friendly_name
        bolFound=False
        #look for the current cc in the known castDevices array
        for cd in castDevices:
            if cd.friendly_name == strFriendlyName:
                bolFound=True
                print(datetime.datetime.now(),"---------------------")
                print(datetime.datetime.now(),cd.friendly_name)
                print(datetime.datetime.now(),"Current volume: ",cc.status.volume_level)
                print(datetime.datetime.now(),"Saved volume  : ",cd.volume_level)
                break

        if not bolFound:
            #this is a new device, need to save it
            cd=castDevice()
            cd.friendly_name=cc.device.friendly_name
            cd.volume_level=cc.status.volume_level
            if cd.volume_level == 1.0:
                #max volume found, reset to default value
                cd.volume_level = 0.3
                print(datetime.datetime.now(),"Set ",cd.friendly_name," to default value: ",cd.volume_level)
            print(datetime.datetime.now(),"Added new device",cd.friendly_name, " volume: ",cd.volume_level)
            castDevices.append(cd)
           
        # update volume if different   
        if cd.volume_level - cc.status.volume_level != 0 and cc.status.volume_level != 1.0:
            cd.volume_level = cc.status.volume_level
            print(datetime.datetime.now(),"Saved volume updated to: ",cd.volume_level)
           
        #Check to see if volume is maxed out and reset it to saved value
        if cc.status.volume_level == 1.0:
            cc.set_volume(cd.volume_level)
            print(datetime.datetime.now(),"Volume reset to saved value")
            time.sleep(30)
           
        cc.disconnect

    del chromecasts
    stop = True

    # persist to file "pycastvolume.csv"
    f = open("pycastvolume.csv","w")
    print(datetime.datetime.now(),"Saving ",len(castDevices)," devices to file.")
    for cd in castDevices:
        f.write(str(cd.friendly_name)+","+str(cd.volume_level)+"\n")
    f.close()