It's only useful to think of ym- as reflexive (doing something to yourself) sometimes. There's this post from Reddit, reposted here:
The usual way of conveying reflexive meaning in Welsh is through pronouns, just like English "myself, yourself etc.":
fy hun(an), dy hun(an), ei hun(an), ein hun(ain), eich hun(an/ain), eu hun(ain)
The shorter forms tend to be found up north, the longer ones down south.
Roedden nhw eisiau lladd eu hun(ain) "They wanted to kill themselves"
Pam mae hi'n bwrw ei hun(an)? "Why is she hitting herself?"
Gwelais i fy hun(an) yn y llun "I saw myself in the picture"
The prefix ym- can have a reflexive meaning but, unlike in other related languages (looking at you, Cornish), it's no longer really productive. By this I mean you can't use it with any old verb to create a reflexive verb.
Rather, I'd look at ym- as a prefix which sometimes has reflexive meaning and use this as an aid to remember new vocab you come across. So golchi > ymolchi is an easy example: "wash yourself". gweld > ymweld is more reciprocal ("each other") than reflexive ("self"): "see each other i.e. visit". And then with stuff like lladd > ymladd it doesn't work; so in this case either think of it as "(nearly) killing each other" or learn it as a totally separate vocab item (I find the lladd connection helps though).
There aren't loads of ym- verbnouns. I think you've listed the most common ones. And more to the point, ym- can mean different things. Just use it as a memory aid when you come across it if it works in that particular case and don't go hating on reflexive verbs!