I would check your battery's voltage drop during cranking. It's entirely possible you have a weak or weakening battery, connections to the starter, or starter motor. Another possibility is that your engine oil is providing unnecessary added resistance because it is too thick and difficult to get flowing on cold start. This usually has to do with either exceeding the manufacturer's spec or using an oil unsuitable for your climate.
If it's the latter, I suggest you use the right engine oil for your vehicle and your local climate. Your owner's manual will have the correct specification.
You can test your battery with a commercially available battery tester. Some battery shops will have them, but you need to see the readings for yourself to avoid being scammed, and compare them with the age and the corresponding labeling on the battery.
