For alternative recommendations – not really quite punk or grunge, but mostly with a bit of grit or attack or attitude about them – how about the following:
Brython Shag – particularly Bywyd Ei Hun (Life Itself)
Alffa – originally basically loud, crunchy electric blues-rock, but these days going for more of metal than blues feel. You could try Gwenwyn (Poison) or Mwgwd (Mask)
Yr Angen – possibly a bit janglier than some of the others, but their first outing, Gorffen Nos, has a lot of quite fast tracks with a fair bit of attitude – and I note that some of the uploads to YouTube come from an account by the name of Grynj Cymreig, which augurs well. Try Gad Dy Wallt Lawr (Let Your Hair Down) or Fel na Fydd e (If it’s going to be like that).
Not so grungy, but I feel still “alternative” - and an absolute belter of a track - is Cyfarfod o’r Blaen (Met before) by Y Cledrau. Thanks to my playing it in the car, it was at one stage my proud boast that my English-speaking elder child could say exactly four things in Welsh: Dw i’n hoffi coffi (‘I like coffee’); panad o de (a distinctly North-Walian ‘cup of tea’); Dw i’n llyncu mul (‘I’m sulking’, but literally ‘I’m swallowing a mule’); and Sut dw i i fod i gogio bod ni erioed wedi cyfarfod o’r blaen? (‘How am I supposed to pretend that we’ve never met before?’) - which is, of course, the chorus of the song.
And, last but not least, I’d come across Chroma before, with one of their English-language tracks (Girls Talk), but I have only recently come across Tair Ferch Doeth (Three Wise Women), thanks to this monster thread on Welsh music, and have since been listening to it a bit obsessively. Drums, bass with a bit of an overdrive (no guitar), and vocals that, on the studio version, at least, have more than a hint of Sugar-Cubes-era Björk about them.
Actually not last: @siaronjames mentioned some older bands, but if we’re going for a bit of a punk vibe there’s a definite garage-band feel to Geraint Jarman’s Gwesty Cymru.
Enjoy.