(If you know Spanish, there's a pretty good degree of mutual intelligibility between Spanish and Italian. IN writing? Much less so, IME.
I spent a year or two as an undergraduate studying Italian. I already knew rudimentary spoken French, and I didn't find that helped, and I didn't know Latin at the time, but I don't think Latin is all that useful for reading literary Italian.
But I did find it pretty easy to pick up, at least at a rudimentary level.)
Yes, I seen rereading that you're not interested in Italian. For all I know you could pretty easily read Italian with your background in Spanish. I only speak a kind of "street Spanish," so I wouldn't know.
(i ]I would probably learn German, at least as a reading language. I find it's a pretty logical language, at least for simpler literary works and, most definitely for didactic works.
Spoken conversational German is a little bit different. The lexicon is, I word say, very large, and very strange, from the perspective of an English speaker. I can understand and speak simple things, but I'm by no means fluent.
One difficulty if you haven't learned Latin is if you're not familiar with the noun cases, but there's only a handful of them, and you can often just infer from context which is being used.
(ii) French is an excellent choice as well. No, you can't exactly draw a straight line from Spanish or Italian to French. The two groups are rather different.
However, in didactic works, it's very unlikely you'd need to worry about much "fancy" diction or complicated tenses.
And, if you're reading it, you have no need to bother about their somewhat odd system of assigning genders to words (but many times it maps to, say, Spanish, pretty well).
There is the advantage that there are more cognates in English than, say, in German, although the latter has quite a few as well. Much of the vocabulary would be somewhat familiar to you.
And, besides, just like in English, there are objective and subjective cases, the inflections are rather simple.
Topic: New Language