Optimal starting class is going to depend on the individual build. Wretch is the evenly balanced one if you want to try different things without being too unoptimal. Vagabond is optimal or near optimal for a lot of different builds, but not if you are using arcane or hybrid magic. Really I would not worry about the optimal starting class unless you are minmaxing an invasion build.

Respecializations cost a larval tear and there are a limited amount, there is no respec glitch.

Dueling meta is 125, general meta is 150, I have heard there is a good amount of action at 200 as well. Invasion builds are every level depending on where you want to invade but tend to stop around 90 to avoid meta level. Summon range calculator I use the equation (rune level) divided by 125, then times 25 to find weapon upgrade level appropriate to rune level for purposes of multiplayer matchmaking. For somber upgrades, multiply the result by 0.4 and round. Not saying this is objectively the best way, just what I do.

If I could play the game again for the first time I would go in completely blind for the best experience. I went in with an idea to do an intelligence build for my first playthrough so I looked up how to find a staff and who to give scrolls to etc. so that I could play the way I wanted to play. And I looked up quests because it is extremely difficult in these fromsoft games to complete quests without missing something or messing it up. But would not do that again if I could do it all again.