Hi and welcome. If that is the Gold series from Monitor Audio, that would be a huge step up from an Onkyo HT package set (and well above my budget
). Monitor Gold not at all in the same price range as Jamo. Maybe you were thinking of the Monitor Bronze 5G? The 5G and 4G line is discontinued; did you find them used? I have not read too many good things about Jamo. At only $700 for 5 speakers plus a sub the quality is not going to be that great and I can't say that it would be an improvement over the Onkyos. Monitor Audio is quite good when you get to the Silver and Gold lines but even the Bronze in a 5.0 package will cost a lot more than Jamo.
What a number of members will tell you here is that if you have a limited budget (under $1k) then put your money into 2 good main speakers or maybe 2 mains and a center. That is where you will notice the biggest difference. Post what your budget is and if you're outside of the U.S. and others should have some recommendations. Here is a review of the current Monitor Audio Bronze setup, but the price includes a sub which you don't need.
https://www.soundandvision.com/content/monitor-audio-bronze-200-surround-speaker-system-review
Whether it is worth it is a matter of your current level of satisfaction and what your expectations are. Can't answer that for you. I would recommend you visit a store and do some listening tests. Even if it's just a Best Buy. If you notice the difference moving to a tower speaker or a better set of bookshelves then it may be worth considering.