What's a good budget Android phone?

My current phone is an ASUS Zenfone 3.  I have no issues with this phone. The battery just wont hold a charge anymore. Ideally i'd just have the battery replaced and keep this one. But given it's age, that's not really a good option. Most of the phones i've been looking at to replace this one, i don't really like. the newer Zenfone line up, the specs aren't much better then my old one. As much as i like the Zenfone experience, i can't justify the cost of a new one without any notable gains. I don't want a large phone either, i want to stay under 6". Out of everything i've looked at, i'm leaning towards the Pixel 4a. But i don't like the fact that it's all plastic, and also lacks an SD card slot. However, the hardware specs are very good for the price. I'm on the fence. I want a phone that's under $400, that doesn't suck. Any suggestions?  It doesn't have to be a new model. I don't mind buying an older model phone in order to save money, as long as it's still a decent phone. I haven't purchased a new phone in a long time. So i'm a little bit out of the loop. What would you recommend? And don't say an iPhone. I can't stand iOS. :)