There used to be a twice a day (or was it once a day ) from Fengyuan to Hualian. And it took near 7 hours as i recall. Going by bus to Hualian is not going to be fast. Those mountain roads are not fast.
As iv said, doing Hualian/ Taroko and back in one day makes little sense.
Unless you are young and have only one day to do this. In which case you may want to (since you do want to save money) take the overnite train from tAichung , which arrives in Hualian early in the morning. Then you can bus over to Tienshiang and spend some time there, then take the afternoon/evening train back to taichung?
but you may not sleep well on the overniter train and you may be a bit zombie-ish.
but i did something like that before when young and you can do it , but its not preferred by any means.
you want all your energy with you when you visit the gorge, especially if you want to do any trails.
what i did a couple of times was to fly down to Hualian and visit the gorge and then fly back in the afternoon. That was very workable.