Tldr; take offer, don’t quit engineering yet, you are fine
Don’t quit engineering if you enjoy it. If you have better offer and the current ship is leaky as fuck => jump the ship. Saving the leaky ships should be very profitable if it is not => you are being heavily exploited.
I jumped the ship thrice. And one time accepted a lower payed position, just because I was quite burnout.
On the topic not using the progress and not understanding the Intenals. Understanding internal will not make you senior. Understanding what you can apply that you already know can make you senior. I remember being in a situation like yours. I thought I didn’t know Jack, but then on a newplace I seen people who were running around like a headless chickens on crack. This has given me a good understanding about what knowledge is and that applicable knowledge is the key.
Consider to take look on http://intive-fdv.github.io/DynamicJasper/ It is more code friendly Jasper wrapper to provide reports. Plain jasper is very XML heavy and you will end in wrapping it in some template engine at some point to reduce repetition. Otherwise download the Jasper Report studio crate simple reports and play around. There are maven and gradle build plugins that compile reports during the build and then you can work with compiled versions.
Jasper by itself is not a bad technology and work quite good for designing reports.