What are the options for extending my stay in Thailand until April next year with a tourist visa?

If you are in Chiang Mai also consider doing a border run to Mae Sai (Myanmar border). This is quicker and cheaper than flying to Udon and going to Vientiane- it is 4h40 on a comfortable bus direct from CM, so you can do the whole thing there and back in one day.

You can only get two visa-exempt land entries per calendar year, but this would reset Jan 1 so if you don't have any yet, doing one in December would not take anything away from your quota.

You get 30 days doing this (rather than the 60 you would get with a visa) and you can extend by 30 for 60 total (rather than 90 with a visa+extension).

If you do a run on Dec 6 you would be stamped until Jan 4, with extension until Feb 3.

You could then at that point either do a visa run to Vientiane for a further three months, with extension, which would bring you through to the start of May, or alternately do another bounce at Mae Sai which would, with extension, get you up until Apr 3.

If you need until end of April, 1 Tourist Visa, extended + 1 border bounce, extended, would get you there. If you are doing that combination, it is better to do the border bounce first, in December, as that will reset on Jan 1 and doesn't "cost" you anything from your quota.