Personally I'm just testing on my Galaxy Note 8, which is quite an old phone now, running an old version of Android.
I occasionally test on my partner's newer device too.
I've got a lot of open beta testers that can tell me how my app looks on different aspect ratios, and how it runs on weaker devices - I haven't really felt the need to buy more devices for myself. Most of the crashes and problems I've found while testing seem to affect all devices similarly. There's only been a few minor quirks that seem to be device-specific, and I've given up on making my apps run perfectly on every single device - there's just too many!