Now that it’s at the end of October, andthe leaves have turned red. Now that it’s the end of October, the leaves have turned red.

"leaves" should have "the" in front of it This sentence would be incomplete if you used "and" here, it feels like there should be another clause after "the leaves have turned red".

Now that it’s at the end of October, and the leaves have turned red. Now that it’s at the end of October the leaves have turned red.

InAt my university, there are many broadleaf trees, like cherry blossoms, and their red leaves are now very beautiful. At my university, there are many broadleaf trees, like cherry blossoms, and their red leaves are now very beautiful.

"At" feels most appropriate here "Cherry blossom" should be plural

However, at the same time, many leaves fall and scatter across the road, which means that workers have to clean up the fallen leaves from the early morning. However, at the same time, many leaves fall and scatter across the road, which means that workers have to clean up the fallen leaves from the early morning.

I'd recommend combining with the following sentence since it describes the result of this sentence. You can combine with a phrase like "and, as a result," or "which means that" to indicate the cause-and-effect nature of the relationship. The repetition of "at my university" isn't needed "many workers" does feel a little unnatural in this sentence so I have changed it to just "workers". This does lose the nuance that there are a lot of workers, but I couldn't think of a good way to convey that naturally here without fundamentally changing the sentence.

In my university, many workers clean up the fallen leaves from the early morning.

I recommend combining with the previous sentence as this is a result of the previous

In my university, manythe workers clean up the fallen leaves from the early morning. In my university, the workers clean up the fallen leaves from the early morning.

When I arrived at university before 8 a.m. today, I saw they were collecting fallen leaves into garbage bags. When I arrived before 8 a.m. today, I saw they were collecting fallen leaves into garbage bags.

Since we already know you're talking about your university, you don't need to repeat it

When I arrived at the university before 8 a.m. today, I saw they were collecting fallen leaves into garbage bags. When I arrived at the university before 8 a.m. today, I saw they were collecting fallen leaves into garbage bags.

I recommend that you visit at least once, though there are quite manya lot of tourists. I recommend that you visit at least once, though there are quite a lot of tourists.

Looking at your original text it seems like you intended to say that you recommend visiting it at least once "at once" means "immediately" (すぐに, etc) "there are quite many tourists" feels a little unnatural, so rephrased it to "quite a lot of tourists" (note that this is probably a bit of a British way of phrasing it, I think Americans might phrase this differently)

I recommend that you visit at oncesoon, though there are quite manya lot of tourists. I recommend that you visit soon, though there are quite a lot of tourists.