I don't want the fuss or expense of an Android or Apple watch. Garmin makes some nice models but they are still relatively expensive.
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.
For heart rate training in general, the gold standard is an ECG-based chest strap. Avoid an optical-based monitor because you get some lag between readings or it even misses readings (this is especially evident during short intense intervals).
For a chest strap HR monitor, the main one is Polar H10, for a cheaper monitor you can get a Coospo H808S.
Saying that, I mainly use an Apple Watch Ultra 1 because it is convenient, it is good enough and I do not have to hassle with putting on a chest strap - I just have to settle with it taking a few seconds to register the correct heart rate after an intense set.