In another post, I implied that (as an Englishman) this should be none of my business really - it’s up to the Welsh to decide whether their language will live or die.
But having said that, I’ll comment anyway:
My vision is of a properly independent Wales, which means it looks after its own finances and has its own currency (which could include local currencies).
If the economy was less dependent on the English speaking world, there would be less feeling that one had to speak English in order to survive and prosper economically.
The same applies to Scotland IMHO, although the language situation is somewhat different there.
You may say: look what happened to Ireland: they have been independent since 1922 (Irish Free State), and in spite of government support, the Irish language is not in a particularly healthy state. Well yes, but I would say Ireland’s economy remained dominated by that of Britain for a long time, and they kept their currency more or less pegged to the pound sterling. Then they hitched their wagon to the EU and the Euro, with initially good, and then later disastrous results. So in a way, I question how independent they have been, and of course, Ireland haemorrhaged vast numbers of people to English-speaking countries, so obviously a need to speak English was always an expectation.
I’m not of course saying that Welsh people should not learn English - that would be silly. But what I am saying is that it should be gradually possible to make it just as economically valuable to speak Welsh as it is now to speak English. I did say this is part of a “vision”, so we are not talking about anything like this happening any time soon, and probably not in my lifetime. But I think it would be kind of cool if Wales and Scotland became the new “Celtic Tiger” economies, leaving England trailing behind, and it was English businessmen frantically trying to learn Welsh and Gaelic in order to do business over the border, rather than the other way around.
And by the way, if having your own currency is seen as a barrier to success, it’s worth looking to Scandinavia which the OP mentioned. Although they are culturally and linguistically very close, very wisely they have kept out of the Euro and don’t even have a currency union between them (which you might have expected) - they each have their own Kroner, and the exchange rates differ quite widely.
If you give up your currency, you give up your freedom, as the Greeks found out. By the same token, if you want real independence, you need your own currency, and I’m surprised the canny Scots aren’t seeing this yet.