If you're getting repeat spoke breakage, it's likely that the spokes have (the broken ones) reached their fatigue life, or are approaching it (the yet to break ones). More spokes will break and it's a pain to have more than one spoke break when far from home - it spoils the ride.
Taking a 300 mile perspective the wheel is beyond economic repair. The rim may still have life: this will depend on its lifetime of riding style, wet/dry weather proportion use and the nature of the grit/sand it's had to roll over (and brake in). If not, 'tis
@fossyant new rear wheel time. With OK rims, you could respoke the wheel economically if you did not incur costs for labour (use
double butted spokes). Otherwise, again, it's new wheel time.
get a new wheel. (Wouldn't know what 2 get or where to start 4 this option)
This is a '700c' disc wheel taking 40mm tyres (so you want a rim which is 622-23) with a 9speed Shimano freehub and a quick release (QR) axle. When buying it may not come with rim tape - you could use the tape from the current one. Rim tape is normally sold in a pair. I'd advise (given these wheels have done many miles of sterling service) replacing the rim tape both front and rear. You want to get a hub with the same disc fitting as your current so you can switch the disc over.
Maybe this bike came from
Halfords. They advertise this wheel for sale:
https://www.halfords.com/cycling/bike-parts/bike-wheels/rear-700c-bike-wheel-in-black-244124.html (but check the disc fitting). This is not a recommendation for this wheel.
