Hi Jack.
This line is from your extract :
[1163836.707190] Out of memory: Killed process 26811 (immich)  ==> this matches an abnormal behaviour of Immich during big import of external library... I have already noticed that I get some troubles with Immich since I use it , I reduced memory amount of my 2 running VM, and still getting some VM crash since I use Immich.
But this has nothing to do with certificat renewal. Please pay attention to timestamp : its obviously a relative timestimp :13 day from "starts" . Absolute start is in 1970.. No chances that this is 13 days after 1970. More chance that this is 13 days after NAS power on or reboot.
Anyway : 13 days after "something"... but if you check system log, you will see that I have rebooted NAS several times in order to try get Letsencrypt working so letsencrypt errors are short after NAS reboot ( not more than 1 day ) , and also that all my tests where done  before Immich was installed.  I installed Immich on 22 of september, and this case was started on 19 of september.
See attached CSV :
> line 370 : reboot 19 of septembre at 1.00 morning.
> line 350 : My last fail while trying to renew certificat, on 19 of september at 10.30AM ( so less that 1 day after last reboot )
> line 311-310 : same day 5.00pm that's at this time that I disabled Firewall, and sucessfully performed renew of certificat.
Currently my certificat is valide until 2025.12.19 3.02PM... that exactly matches the time of renewal + default validity period given from Letsencrypt ( with local time / UTC time I suppose ) .
Why don't you try to reproduce the issue at home? Use a domain other than Asustor (because it works differently with Asustor DNS), use my firewall settings, and try to create the certificate with the firewall enabled.
best regards.