What does 都挺 indicate here?l
In reading Chinese, we are always faced with the difficulty of correctly grouping characters. Here the correct pairing is not 都挺。 Rather, you should view 一直都 as one group, meaning all along and 挺 as a modification to 想。The meaning of 挺 comes very close to 很.
Examples：

我一直在等你/I’ve been waiting for you all along.
今天挺冷。/It’s pretty cold today.

I would say there is virtually no difference between 一直 and 一直都. The choice is guided by rhythm.