We are thinking to use our Jira Cloud which is currently used for the development also for the CRM. CRM means in our case just a data base of customer calls and mails with our replies. No large database connections or bug tracking (this is handled besides in a different Jira project) is required.
The question is, if Jira Cloud can handle 1.5 Mio. customers plus 200 k dealers multiplied by (several) requests per customer.
The idea is to keep it as simple as possible and just create an issue type "customer" that relates to (multiple) "requests". The only requirement is that our support team can search for the customer quickly and also search all issues for similar problems in the past.
I already found this link (https://community.atlassian.com/t5/Jira-questions/Does-JIRA-Cloud-slow-down-if-there-are-a-large-number-of-issues/qaq-p/913627) that tells that there is no limit but I'm afraid that usually nobody works with > 3 Mio. issues?