Is ongoing proof of funds required for the DTV visa in Thailand?

That 11k is supposed to show that whatever happens you in no way whatsoever need to rely on any income for the 6 month period

Makes sense that you can't live off the £11k for 5 years

For a 90 day check in with a DTV what do I need ?

It would perfectly make sense to have to show funds again for an extension. The opposite would be surprising as it would make the requirement when applying kinda moot.

As far as showing proof of funds for a report after a new entry (If I understand correctly the wording of OP) that's sketchy and maybe something has been misunderstood by someone or lost in translation.

Showing bank funds it's not required. Source: hundreds of DTVers come in and out every day, and they always get 180 days, no questions asked.

Not saying they couldn't change something in the future, tho.

If I understand I can stay for 90 days, leave the country and back again for more 90 days?

It will be up to each immigration office to determine the requirements they see fit to ask for.

No one knows what the requirements for a 6 month extension will be yet because no one has done one. But each immigration office can require whatever they want.

As far as leaving and returning, the immigration office has nothing to do with that. You'll have a new 180 day stamp that your immigration office can't do anything about. They're just making stuff up.

If you're actually worried about it, then just don't do 90 day reports. If you never try to do an extension and just do border bounces every 180 days instead, there won't be anyone to ever notice you didn't do a 90 day report. And worst case the fine for failing to do it is 2000 baht.