2nd hand Garmin or Apple Watch. These things are basically being thrown away and watches like a Gen 6 AW or a Fenix 5 can be found for so little money and they work very well even today if you only care about sports tracking.

I personally cannot stand Apple for sports, which is why I have a AW Ultra 1 AND a Fenix 7 lol. Got both 2nd hand for ~30% of their MSRP.

Personally something like a 2nd hand Garmin Forerunner 55 with a chest based HR monitor is for me the best budget setup. But when shopping second hand you, use what you can find. I often see great deals on things like the Garmin instinct series.

2 years ago I got a Fenix 5 for 180 and it was amazing for me. Just turn off all the "smart" features.

I would look into a used entry model Garmin. Something like a 235 (I have my old one sitting around). It has an HRM though it's not a terribly reliable one.

You could just make a playlist of your intervals and use earbuds, or exercising in the town square under a large clock