what do I replace an Airport Express with, to extend a wireless network AND stream music?

So I'm thinking it might be the Express. It is about 8 years old. What would I get in 2020 to replace both the wifi extending function and the Airplay music function?

Split the functions. The Express will probably still work ok for airplay. What it is poor at is wireless.. you can use any wireless Access Point (AP) or wireless router in bridge or WAN BYPASS mode. i.e. a router may not have a specific AP mode but all wireless routers can be used as AP. They can be exceedingly cheap if you are prepared to look around. I buy a Netgear Cable modem router which can be set to WAN BYPASS and costs down to $1. It has excellent AC wireless about the same as the R7000 of a few years ago. New AP / Routers can vary in price quite a bit. I am in Australia so what I use here may not be relevant to your market.

You can then plug the Express into the AP and use it for just airplay in bridge. That means you do want an AP with at least 2 ethernet ports otherwise you need a switch as well, so a router is often a better choice.

What would I get in 2020 to replace both the wifi extending function and the Airplay music function?

One idea.....maybe not a permanent fix, but good for now........buy another AirPort Express. New, Refurbished and Used AirPort Express(es) are available on Ebay.

it's not a replacement but you can get many bluetooth devices which has analogue 3.5mm minijack output and or RCA which one can connect to speakers or a stereo

it would not support many speakers like airplay and the quality of the audio would like not be as good

this is the one I have https://www.cnet.com/reviews/belkin-bluetooth-music-receiver-bluetooth-wireless-audio-receiver-series-review/

it connects to my sony receiver

AFAIK, there is currently no device out there that can perform the same functions as an AirPort Express. That is, perform as a wireless Internet router, or extend a wireless network provided by another AirPort base stations, AND support AirPlay for streaming.