I'm making a sequel to one of my games. I found that having a finished game to build on doesn't save nearly as much time as I thought it would.
Some things like the save/load system and menus I can use with very few changes.
Other things that I need to change actually take longer to adapt than they did to implement originally back in 2019. Plus I'm stuck doing things in stupid ways which was the best I could 6 years ago.
My professional game dev tip would be to make some standalone systems with reuse in mind (duh!) and then to not even try to adapt/build on anything else and instead just make it from scratch using the old code as a loose reference.
inb4 that's all obvious, you're too stupid to give advice. well okay but in case anyone's even dumber than me