Pitch accent of "たくさんの人"

As u/Zarlinosuke notes, this is starting to get into phrase/sentence-level pitch accent rules. Unfortunately, OJAD and other automatic tools aren't really reliable in this area yet.

There are two phenomena at play here: the pitch accent of たくさんの and the pitch accent of 人. The latter is less complex, so let's deal with that first.

人 is normally a 平板 word -- i.e., ひと￣ in NHK notation. However, as u/whateveranywaylol notes, when qualified by an adjective or other attributive phrase, it becomes ○○ひと＼ -- i.e., 尾高. Other words that follow this rule are 日（ひ）・上（うえ）・下（した）. So OJAD is correct in inserting a downstep after 人.

Now, onto たくさんの. の affects pitch accent in somewhat complex ways, and not all sources or speakers agree entirely on the rules here. Dogen presents a good synthesis of this topic in lessons 60-61 of his Patreon course, and you can consult the sources that he cites -- e.g., the 新明解日本語アクセント辞典 -- if you want to research more thoroughly. の can, in some cases, delete a downstep in the final syllable (yes, syllable and not mora) of the previous word, which is why 日本の is often にほんの￣. This appears to be what OJAD is trying to apply.

However, most sources agree that downstep deletion cannot occur with counters or other nouns of quantity, and たくさん is specifically cited as an example. So this should be たくさ＼んの and not ×たくさんの￣.

Putting this all together, the pitch accent should be たくさ＼んの・ひと＼が.

Again, this is a fairly complicated topic, especially around the treatment of の.

Also, as a side note, OJAD is complaining in your second screenshot because you put a downstep on the particle が, which never occurs.

In general, I'd say that sentence pitch overrides word pitch, especially when it's being spoken at a natural pace--so my sense is that your program is right, but I'd be interested to hear if anyone disagrees.

So, not an answer to your question really (I'm definitely a pitch accent beginner, though do remember learning somewhere that particles can change pitch accent occasionally), but when I'm worried about various voice synthesis tools, I always check YouGlish:

https://youglish.com/getbyid/28111/%E3%81%9F%E3%81%8F%E3%81%95%E3%82%93%E3%81%AE%E4%BA%BA%E3%81%8C/japanese

Anyway, how it helps.

Not sure about たくさん, IIRC it's nakadaka as a noun and heiban as an adverb, so I would expect nakadaka here, but 人 is odaka when it's modified.

たくさん is [0], I think.

It's [3] when it's used like, "もうたくさんだ."

人 is also [2] in cases like "たくさんの人."

It's [0] when it's like, "人を呼び."

I checked with a dictionary and it says the same thing :)