Which is more important? Servant Skills or Level?

I focus on levels then skills, but if the servants a support servant prioritizing skills over levels isn't wrong.

Take your waver for instance, with his growth curve he only gets about 200-500 on his stats between levels 80 and 90, so leveling his skills and leveling a servant who'll get more stats is better.

Also lots of skills in the game can be safely left at 6 if you don't plan raising it to 10 as the they don't get a shorter cooldown until then and the difference in power between level 6 and 9 is quite often the same as the difference between level 9 and 10.

Both is important. High level = to higher damage/HP.

For skill, sometimes at level 6 is already good enough but for important skills like Tamamo fox wedding and Jack murder on the misty night best max it.

Definitely level cap so you at least have access to all 3 skills. Then aim for at least 6/6/6 for servants you intend to use and have as supports.

Skills takes much more investment and farming than levels. Servants with higher levels will last longer in a match, so I'd say prioritise levelling the servant before the skills. If you want to be extra efficient, I'd say just don't bother farming skill materials at all, just wait for them to come in the event shops. Farm embers until you've levelled your servant sufficiently (lvl - 70+) then farm QP. Only when you are bored of doing one or the other, then farm materials because outside of events the drop rate for them are awful and imo an inefficient use for AP.