Transit visa at Bangkok (BKK) for non-code share airlines

YES, separate itineraries require a fresh check-in of baggage on international flight and most domestic flight.

If your flights are on two separate itineraries, they are separate tickets. If they are on the same itinerary, here are some clues to confirm:

1. Contact the first (or each) airline and ask them if the luggage will automatically transfer to the next flight. If you used a booking/travel agent, contact them and ask them. This would be your best source of accurate info.

2. Check the itinerary online directly with the airlines you will fly with. They usually require the booking confirmation code and passenger's name. Do all the connecting flights show as one itinerary?

3. Do you get both boarding passes when you check in? Or only boarding pass for some of the legs?

If you bought the tickets yourself, the general rule is that if you bought the legs separately or were told when buying that each leg is separate, then you must assume that you will have to get your luggage and check in again (which requires passing through customs/immigration).

....Nationals of certain countries may apply for visa upon arrival in Thailand. Travellers with this type of visa are permitted to enter and stay in Thailand for a period of not exceeding 15 days. For more information, please see Summary of Countries and Territories entitled for Visa Exemption and Visa on Arrival to Thailand......

According to this list from the Thai Ministry of Foreign Affairs site, you Mainland China nationals can get VoA for 15 days.

Here is a screenshot.

During peak times or if immigration system is slow, it could take 1 hour or more to pass through Thai immigration. This is excluding VoA, or re-check in time. With check-in baggage, and factoring in other delays I would recommend 5+ hour layover gap.