> 50-53C is common to avoid risks of scalding but some are set as low as 45

Wait what, isn't the minimum temperature where Legionella will die around 60°C? Are you talking of a country where they (noticeably) chlorinate the water? I thought that was very uncommon, except for Southern Spain and Italy. If you set your boiler to 45°C in a country with (nearly) unchlorinated water you'll have a nice Legionella culture after your 3 week vacation.