You're in the 50+ forum, but how old are you? It makes a difference in how you can most profitably train.
I have used a HRM from 1995 to the present and a HRM and a power meter since late 2019. I've used a chest strap and several generations of Polar watches. On the bike, the watch was held by a Polar watch mount. Now I use a Garmin which can display both HR and power.
The weakness of a HRM is that it responds slowly to effort which makes it tricky to use for intervals. The strength of a HRM is that it shows physical stress. That allows it to show more than power. Its display of effort can be modified by exhaustion, dehydration, and low blood sugar for instance. Thus a HRM can present more and more difficult to interpret data than a PM. It takes a lot of riding to learn how to interpret the data presented. Adding a PM makes that interpretation much simpler.
HR is a analogue of oxygen uptake. When your muscles need more or less oxygen, they tell the heart to change its rate. An increase in breathing rate will always precede an increase in HR. That means that breathing rate is a good analogue for HR. There are really only 3 training zones, below VT1, above VT2, and between VT1 and VT2 (google). Those zones are defined by breathing rate, thus locating the HR at these zone boundaries is the first order of business with an HRM.
I upload my date to my TrainingPeaks Premium account for data viewing and analysis. There is other similar software. I find that data increases my enjoyment of the sport because it can answer what's always a big question in training, WHY?
I follow a year-long training plan produced by ancient software which is no longer available. I have tried the usual 12 week programs and found them inadequate for this geezer's needs. 12 weeks is about the time I devote to my fall base training. I start in October looking for a July peak, so I'm just now starting to ride hard. I recommend reading Friel's Cyclist's Training Bible and Fast After 50 to get a good understanding of the training process. That understanding needs to come before embarking on any sort of training plan.
The comment above about riding lots at a moderate intensity is a good point. The intensity recommended is below VT1, so both your breathing and HR will show you where that is. However I live in hilly terrain and am nowhere near as strong as that poster. Thus I can't do that sort of riding outside at all. Outdoors, I ride in all the zones, will I or won't I. I work on the steady moderate pace indoors on my rollers.
I agree the focusing on speed and distance can make one stale. One needs to focus on one's uploaded data. Big difference as you will hopefully see. I highly recommend getting a chest strap for accuracy. I also recommend using Heart Rate Variability (HRV) to assess one's training state. I used to just use morning resting and standing HR tor training state, but after my 60s they became pretty much useless. Old hearts just aren't the same. Now I use those resting HRs plus HRV. Yes, PITA but self-evaluation is the key to successful training. At least numbers give you a reason for why your legs don't work on a specific day. Helps the cyclist's stone hard head to decide to take it easy after all.
https://elitehrv.com/heart-variabili...tible-monitors
Everyone's different: I watch my power, HR, and cadence, nothing else (never watch your speed), but not so much that I don't watch the road. One had to be careful about that. I have a riding buddy who rode right into the back of a parked SUV while fooling with his Garmin and sustained serious injuries.