So I know that there are topics about cars that start badly or don't start, but given the fact how many videos I watched and how much was my car diagnosed and troubleshooted on is giving me headaches so maybe some wizard would be able to tell whats going on.

So let's start with the car itself. It's Mazda 6 2004, 2 Liter, Diesel, Manual, 280k km. Had for two years, changed timing belt with my dad and some other parts. Well maintained with good oil and filters. The tires are worn and there are some other issues with rear lights but nothing too bad.

So let's get to the problem. So at first it sounds like a simple issue with the start up system of the car, but it's not. At first the car would hesitate to start and it seemed like it could be a dying battery so the battery was pretty old and winter was coming by so I bought a brand new strong battery for like 100 bucks or more. The hesitation to start persisted and some days after driving more than 8-10 km. the car would just not start and would crank really bad and if you disconnect the battery for 5-10 minutes it would start again most of the time. So we thought let's change the starter and that pretty much did nothing and it only maybe got better with that hesitation to start and it went from like always hesitating to start to like hesitating to start 50-60 % of the time. I went to a mechanic to run obd diagnostics and he told me that there is missfire on one of the cylinders, but that doesn't explain why the car would sometimes crank like a new one and sometimes like barely. The other day we figured that we should check the battery when the issue happens with bad cranking or no start. The battery seems to be dead and only after you disconnect it and reconnect it then it starts. The last troubleshooting still needs to be confirmed tho because we did not have a tester to see battery voltage and we could only see the indicator on the battery. Dunno if it helps but alternator shows 14.4-14.6 Voltage charging with all the lights and stuff on. Diesels don't have spark plugs so there could be issues with fuel system and maybe injectors, but that issue was ruled out because when the starter kicks the car just starts good.

Since it seems like an unsolvable issue I'm planning on buying a newer car 2008-2014 lexus/toyota and would appreciate if someone would recommend a good OBD scanner just to check before deciding to visit the mechanic for an actual check up.

Thank you for any answers or other troubleshooting that we could do. Maybe it's the computer that is going bonkers or some electrical gnome living in the engine bay?

EDIT: the car can be left alone for days and it would start all the time with some hesitation.