Zammad can actually work very well without Elasticsearch for small amount of tickets. Please note, after disabling Elasticsearch, you may see many BackgroundJobSearchIndex errors like this, so use it at your own risk.

zammad run rails r "Setting.set('es_url', '')" # set it empty
zammad run rake searchindex:rebuild
systemctl stop elasticsearch
systemctl disable elasticsearch
systemctl mask elasticsearch 

Further reading: Set up Elasticsearch – Zammad Docs