I am looking forward to an app that gives minimal features of Jira with inside of Nextcloud.
or if any other existing app someone is using ?
I think there is no app like Jira in Nextcloud. The apps in Nextcloud are mostly small.
I think you need to think about what software you need minimally. If you need Jira than you need Jira. For alternatives search the internet “alternative jira”. Then describe what you really need and maybe there is a software solution also included in Nextcloud. But i think not. Nextcloud can also not yet wash clothes and cook coffee. 
For minimal features maybe you like deck. But this is very minimal. Search https://apps.nextcloud.com/.
replying here to check if anyone has done something good with finding a good combo to replace JIRA + Confluence?
Of course Collectives should be OK as Confluence alternative, for JIRA part though it is a separate thing indeed - I guess partly can even be done by Deck, but it’s not really the same
good old Bugzilla perhaps ?? 
There is an OpenProject integration that I have been investigating this week. I haven’t actually used Jira, but I think it may be something on the same order. Of course, OpenProject is a separate install, but it seems that it can use Nextcloud for file storage and there is some dashboard connectivity.
Openproject+xwiki would be the opensource alternative, lets see here: Strong open source Jira and Confluence alternative: OpenProject and XWiki join forces
There is a Nextcloud Integration of openproject (to access files from OP, see open tasks).