Macbook is waking up from sleep while on wifi with iMac
Power Management is the culprit. 

You should use the pmset command to see what power management has been doing.  In Terminal, the basic command is

pmset -g log


But that gives you all the power management events since last boot time.  You can reduce this with judicious use of grep.  I have used both of these (with similar results):

pmset -g log | grep -e "Wake from" -e "DarkWake" -e "due"
pmset -g log | egrep "\b(Sleep|Wake*|DarkWake|Start)\s{2,}"


Power Management has about 20 settings that can be changed by an admin user.  To view the settings (they will be different when on battery and power):

pmset -g


For my issue, which looks similar to yours, I changed the tcpkeepalive setting.  With this enabled (value = 1), the MacBook will frequently wake up due to network events.  

To change turn off tcpkeepalive use the Terminal command:

sudo pmset -b tcpkeepalive 0


The -b is to set for when running on battery.

I have also tweaked a few other settings like standbydelayhigh, but this is purely tweaking.

This is based on my experience with a MacBook which would lose too much (I thought) battery whilst closed overnight.

You may find these two links helpful:
How to Fix macOS Mojave Battery Draining Issue and 
man pmset.  

Note that I have not changed hibernatemode from the default value of 3.  Many (including the linked article) discuss changing it to 25 which I do not recommend with recent (last 4 years) hardware and macOS version.