What are the visa processes after immediate resignation from a job in Thailand, and how can I convert my visa to a tourist visa?

The date you resign is the date you must leave the country. Other than feeding the baby issue are you on good terms with your employer? It might be worth speaking to them and saying you will need to resign due to the issue but would appreciate a few days to sort things out before doing so, then ask them to provide a letter saying you will finish on a particular date. On that date you'll need to leave the country

I went to cancel my visa before and immigration told me just to cross the border, therefore cancelling my visa and work permit. I came back the next day on a visa on arrival and extended that, then got my new non B after that. People on here said I’d get fined 20,000 baht but I just done what immigration here told me to do and it was grand

You will need to

Leave on the last day of your employment if your on a visa you’ll not need to cancel that but if your on an extension you’ll have to cancel that as well as your work permit no grace period you may apply for an extension and get denied and given 7 days to leave 1900 baht

Are you on the initial 90 day non-B visa or on a 1 year extension of stay based on your work? It makes a huge difference. If you're on the initial non-B visa, a visa cannot be cancelled and you can stay until the end of your current stamp. But if you're on an extension (the stamp in your passport says "extension of stay") then you don't really have any options.

If you're on an extension then your permission to stay in Thailand ends the same day that your job ends. You do not get 7 days to leave the country. You cannot cancel your work permit or extension without paperwork from your company.

If you do not cancel your extension then in the future you will not be able to apply for another long term extension and will likely be fined 20,000 baht for failing to cancel it when you stopped working. Even if you leave Thailand this will still happen if you return later and try to apply for a long term extension.

Your company holds all the power.

The only thing you "might" be able to do is if your company will give you paperwork stating your last day of work, you can cancel your work permit then take your cancelled work permit and that same paperwork to immigration to cancel your extension. At the same time you can apply for another extension even though you are not eligible for any extension. If they accept your application and 1900₿ fee (they do not have to accept it) they will give you a stamp that says "extension denied" and it will give you 7 days to leave Thailand.

You cannot convert to a tourist or anything else.