I like Gareth King’s books (they have a very user-friendly mutation marking system - and his dictionary too, which is why I can find almost anything in it) and also the “Welsh rules” book that have wonderful lists of collocations (which verb is used with which preposition and so on). I also have “Welsh grammar you really need to know”, which is fine and quite clear and understandable.
I personally don’t understand why people seem to dread grammar so much. Grammar is beautiful, it’s the cosmos, as the Greek understood it, which gives form and harmony to chaos:)
But grammar is normally (English grammar and, as I’ve noticed, Welsh too) taught in levels - first something is explained without any complicated detail (just put Dw i’n + verb together and be happy). Then, when the learner’s more confident and has more vocabulary and can say more things, finer detail is taught. It’s not very useful to do 5 tenses with all their exceptions and nuances in one day, because the tired brain will just dismiss the information it doesn’t consider relevant at the moment (everything you can’t use in real speech).
I realized I just wrote with more complicated words what @owainlurch already said.