i haven't tested it myself but this looks very promising as a service desk tool: https://freescout.net/
i sugest focusing more on finding a tool that will help you interact with external users since in my opinion this may be a bigger challenge (even if it doesn't happen often). since your team is relatively small any project management tool will be good enough for you.
If you can use girlab yo may consider using GitaLab issues. It worked well for me in many projects.
If you need something more sofisticated then simple issue tracker and would like to consider using a SaaS product you may try https://www.jetbrains.com/space/ (i particular project management module https://www.jetbrains.com/space/features/project-management.html)
Another interesting SaaS option is notion.so.