My hibiscus tree is 15 feet high but this year is the worst for the centre branches dying. I haven’t pruned them out yet but I do fertilize, water regularly and it usually has a beautiful showing, although the japanese beetles love it but why is there more damage this year than ever before– possibly winter kill was more severe, I don’t know.
I believe that your hibiscus tree is a Rose of Sharon (Hibiscus syriacus). It sound like you are doing everything right to encourge healthy growth of your Rose of Sharon. As this species is quite susceptible to winter kill, that may very well be what has affected the center of your tree given the past winter in this area. Here is a link to a previous Toronto Master Gardener post on winter kill and Rose of Sharon, which you may find informative: https://www.torontomastergardeners.ca/askagardener/rose-of-sharon-2/
It is quite acceptible to prune out the dead wood in the centre of your tree, and this is often done to promote an airy open growth pattern. As the flowers of Rose of Sharon grown on new wood each year, pruning should be done in late winter/early spring. Here is another link, to an article on pruning Rose of Sharon, which you may find useful:https://www.gardeningknowhow.com/ornamental/shrubs/rose-of-sharon/pruning-rose-of-sharon.htm