If you love maths then you're going to get very frustrated doing further... I.e. if you are doing spech and coping then further is kinda redendant in terms of teaching you much more (unless you are just doing it for an 'easy' high mark but there are numerous thread debating how ethical this is/if it will actually help you). The inverse situation being where you are doing spech for the mark up in which case it is not worth the stress if you can't handle it.
Regardless, even if you do love maths and there is a reason for you to do all three VCE matsh subs, be awar only two of these will be able to count in your primary four, the third will HAVE to be in your bottom two regardless how well yous core in it.
Addressing you final point, you really do love maths then i would highly recommend dropping further in favor of MUEP/UMEP as they will really be interesting and extend you if you love maths. Workload is easily do-able if you are prepared to put an hour or two into Uni maths each week.