You cannot upgrade from 18.04 to 19.04.
Ubuntu does not allow jumping over versions while upgrading (except for LTS to LTS). You have to go from 18.04 to 18.10 and then 18.10 to 19.04.
However Ubuntu 18.10 reached its end of life last week. So you can't upgrade to 18.10. And therefore you cannot upgrade to 19.04.
LTS to LTS Upgrade
If you want to continue to use some software specific to 18.04 then do not upgrade to newer versions of Ubuntu. 18.04 is LTS, and you will be able to upgrade directly to 20.04 LTS next year in July when 20.04.1 comes out.
Or you can continue with Ubuntu till April 2023, its end of life. In March 2023 you can upgrade to Ubuntu 20.04 LTS and then immediately upgrade to 22.04 LTS if you want the latest LTS version available then.
No holding back packages through upgrade to new version of Ubuntu
No. You will get the same message you got now
Please install all available updates for your release before upgrading
when you try to upgrade from 18.04 LTS to 20.04 LTS.
Ubuntu upgrade process requires that all packages are up-to-date from the current repository before it allows you to upgrade to the next version of Ubuntu.
It is my understanding that both can be installed on the same machine at the same time. You can use the following command to switch between the two versions of Java:
sudo update-alternatives --config java
This will show you the list of Java versions currently installed and allow you to choose which one to use. Run the same command again and choose a different version if you want to switch.
Hope this helps