MY advice is to save your money. Use how breathless you are as your indicator of being in the cardio zone, it is more accurate than a heart rate monitor. As a guide, try to talk if you find you can’t easily due to needing to get air, then you’re in the zone. The heartrate which matches that is person specific which is why a heartrate monitor is of little value.
Use a stop watch to measure elapsed time over a regular route.
If you use your breath as a guide then you’ll increase your fitness quick enough.
I don’t know your age but the usual problem is not the immediate next step but you will have plateau effects where some part of your body will be a little slower than the other parts to strengthen and your performance will not improve for a period while that part improves then you’ll get a breakthrough. So the issue with being micro-detailed with fitness trackers is you get despair in those plateau periods so cause the opposite of what you want of giving up. So to you avoid that, it is best to NOT measure yourself too detailed.
There are tiers of systems inside your body which strengthen at different speeds. The first one is skeletal muscle which thickens up in the order of a couple of weeks. They are refreshed via your liver which has to physically alter itself which takes a couple of months (known as developing endurance).
I also recommend you don’t micro-monitor your weight, because one of the initial effects of exercise is the muscles thicken up (many reasons, like more sugar is stored inside your muscles) so your weight will go UP at times. It will drop overall but on a week by week basis you may find weeks it goes up as your body adapts to a new habit.