I had an Octavia diesel VRS and currently have a 2011 Mazda6 petrol I've had for a long time. I'm a big fan of the Mazda which is well specced, practical and been very reliable but the diesel Mazda6's (both mk2 and mk3) have known issues although it's difficult to tell how widespread they are since you'll see more negative comments online.
I had Mazda 6 2.2 diesel for 3 years never had a problem very impressed with mazda
Avoid Mazda 2.2 Diesels. They are well known for blocking the oil pickup and seizing the engine due to injector seals. Check Honest John. Personally, I would avoid the DSG (or any similar gearboxes). They need regular expensive services which often get skipped once the warranty has expired. What mileage and type of driving (city, motorway etc) do you do?
Avoid the Skoda with the DSG, its jerky at low speed, even worse when mixed with turbo diesel engine and add on stop start and its not worth the brain ache. The Mazda diesel will need some careful looking at the paperwork. They had an issue in first few years where the oil would rise when hot and leak out. Mazda eventually fixed it with new dipsticks and reprogramming of the ECU. Make sure your isnt an ex fleet that was sold straight off before the patch was applied!
I've been fortunate that my commute is motorway (pre covid 20,000+ miles pa) but it has been used for mostly small journeys for the last year but not noticed any issues. The auto definitely isn't as quick as the dsg gearbox (other half has a polo GTi) but the Mazda is smooth and quick enough if you need it to go :)