The main problem there is that Owain Glendwyr's rebellion is just one of the micro-conflict of the HYW, where the two main belligerents fight by proxy during the long truce.
While Owain and Tudurs, thanks to small wars operations (largely decentralized) and tactics, managed to fend off Lancaster advances in Wales, the support from France and Brittany (but as well the support, would it be passive, from Richard's II former supporters) was really what allowed them to turn a relativelt localized rebellion into a large one in 1403-1404.
It's less the lack of support at this point, than the quality of this support : the 3,000 and so french/bretons/scotsmen in Wales in 1405 were badly supplied, and failed to represent a strategical boost (things might have been different if they managed to capture the castle of Haverfordwest). Which Henry IV was aware, avoiding any direct confrontation with the Franco-Welsh army.
Eventually the problem of French involvement is that Wales doesn't provide much scure suppliment, and this handicap any real campaign against most important castles, and even more in England.
Past 1405, the French involvement was considered too costly (especially as the political tensions between supporters of Orléans and supporters of Bourgogne raised, until going full civil war in 1407) to be worthwile : the point was to force England to waste its ressources and efforts, not the contrary.
From this point onwards, Owain's rebellion failed to achieve its objective, which was to propose an independent, autonomous at best, principalty of Wales; and became anew a guerrilla revolt. In spite of victories in the mid-1410's, it wouldn't go much anywhere partially due to the lack of French involvement, and the lack of support in Britain (Lancaster's hold being clearly affirmed) which open the door of royal pardon, a very tempting offer in face of failure.
What do we need, then, to make the rebellion more successful?
- A decisive Welsh or Franco-Welsh victory against England : if Henry IV somehow wasn't able to prevent the direct battle in 1405 at Woodbury Hill, then it could have changed things, for a whle.
- France not going into civil war : at this point, it would mean that French support goes down, including naval and subsides, as English raids and chevauchées are yet again a thing jn France
- Pro-war policies in France, even if it doesn't makes much sense at this point : it passes trough, IMO, an Orléans victory early on.
- Lancaster's power in England being at least challenged internally. Let's say that Henry IV dies of the illness he contracted in 1405. You won't have a regency, but you could see the rise of discontents and maybe something looking to
an actual identure.
Basically, no civil war in France, and a civil war in England.
Not impossible, strictly speaking, but probably multi-PoD-al.
Now, how long would it survive a greater success is anybody's guess, but I don't think it would be that viable on the long term in face of an England which would be bound to recover after a while.
