Edit: I just realized you have a zero W (not a 2W?). If so, you have a zero W then comparing it to a zero 2W may not be a fair comparison. Still though, I would think a zero W would run weewx fairly easily...
Here is what I have on my rpi zero 2W using the "top" command(see below). You may want to run top on your rpi and compare.
It is currently running both WeeWX and Cumulus MX and the GUI desktop.
To free up more memory, I can disable the desktop. I will be moving a local MQTT broker to the rpi zero 2w as well.
I found Debian Bullseye problematic and reverted back to Buster. Tom's hardware has been particularly critical of Bullseye.
It might be worthwhile to run an experiment: get another SD micro card and flash it with Buster..install WeeWX and run the simulator or something and see if things improve.
--
top - 11:55:29 up 20:29, 2 users, load average: 0.72, 0.70, 0.68
Tasks: 146 total, 1 running, 145 sleeping, 0 stopped, 0 zombie
%Cpu(s): 6.2 us, 2.4 sy, 0.0 ni, 90.6 id, 0.1 wa, 0.0 hi, 0.7 si, 0.0 st
MiB Mem : 428.0 total, 54.0 free, 180.4 used, 193.6 buff/cache
MiB Swap: 100.0 total, 14.5 free, 85.5 used. 177.1 avail Mem
--