Best way to upgrade my old Android phone without breaking the bank?

I've got an Android phone that's about 4 years old now. It's definitely starting to feel slow, battery life is meh, and some apps just crash randomly. I love tinkering with my devices, but I'm not ready to drop a ton of cash on a brand new flagship.

I've heard about things like custom ROMs, replacing the battery, or swapping out storage to breathe new life into older phones. I've never tried flashing a ROM before, but I'm curious if it's worth the hassle and risk. Also, how much can a battery replacement help with performance and overall experience?

Has anyone done a budget-friendly upgrade on an older Android? What was your approach, and how did it turn out? Would love some tips or warnings before I dive in.