Options for extending range/transmission quality of a HT

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I've been eyeing a certain repeater in my area recently, but only just started trying to transmit on it. It's around ~27 km away so a little far, but reception is more or less clear, and I can hit the repeater, only most of my transmissions are mostly undecipherable except a few words. I’m currently using a Yaesu FT-60R with the stock antenna, but I’ve ordered a Nagoya 771 although I'm unsure if it’ll make a significant difference. I know line of sight is important, but as a student I can't really go walking outside far at 9 pm in the dark. In that case, should I keep looking for antenna upgrades or look into an external setup? What are my best options for extending the range and improving my transmission quality? Advice appreciated!