They both connotate a way of thinking. It is my understanding at 思想 os a more broadly encompassing term, like "mentality", or an overarching mindset, whereas "想法" is an individual idea. But what if you want to express something in between, like "We have lots of similar ideas"? This is a bit different than saying, "we have a similar mindset", but it also seems using the work "想法" would be a bit too limited in this case.
If you want to express "We share quite a few ways of thinking", you can say "We have lots of similar 观念", which is short for "思想观念"。
Or "观点" / "看法", point of view.
"想法" is usually used in a sentence like "I have an idea to do something", likely to combine with verbs.
"思想" is a general term.