500k I imagine is funds that you can support yourself with in Thailand for the 6 month period. Immigration could ask you to show funds at re-entry, but they might not. It's Thailand. 🤷
Had ED visa for 4 years, IB and B visas for 10-ish and did 3 tourist stamps before applying DTV and no problems. Safe to say previous visa history doesn't matter at all as long as everything has been legit. Just meet the DTV requirements.