What are the best visa options for a retired person wanting to stay in Thailand for 4-5 months?

Hey all, I am sorted for a visa, but I am seeking some advice for my dad, he's talking about applying for the STV (same as me), but I feel maybe another visa or process would be better for him. He wants to stay in the country for 4-5 months.

My question is for everyone who has travelled on the TR visa or Visa excempt, how are you managing to stay in the country for longer than the 2/3 months which these visa's/excemptions allow? Also, is there a more suitable visa for a retired person? (note- he's unmarried, doesn't live in Thailand but spends many months each year there)

many thanks.