I’m planning a trip fronm Lyon (France) to Härnösand (Sweden) by hitchhiking.

I’ve got very dgreart friends there whom I used to visit more than once a year.

Your sentence is correct, but it's very formal, especially "whom" which is very rarely used in practice, even though that's "correct". So I took the liberty of rewording slightly.

https://youglish.com/pronounce/whom/english

Fortunately I met some of them in other circumstances but I haven’t met the other ones for a very long timeit's been a really long time since I saw the rest.

This year, I’m not richer than beforeI was, so I thought that it could be a great adventure to travel there for free, by hitchhiking.

I think I cwould find it demoralipressing to make this kind of trip by myself though, on the contrary, but on the other hand, I’d find it very exciting to do that with a dear friend.

not sure what you mean by demoralising, usually that means you feel like you want to give up.

"I found it very demoralising that no one understood my broken French"

I think I would have said "depressing".

But maybe you mean you would find it difficult to finish the journey? In which case it's probably fine.

I’m so, so, so, so happy abouthat thatis friend is coming with me!!!

I really love this guy and I feel amazingly happy to spend so much time with him, to go through these “difficultiovercome these challenges” with him, and to introduce him to my Swedish friends… It’s going to be such a great trip!

"difficulties" sounds like you have problems that you can't control, for example "financial difficulties" etc, but in this case it's an adventure you are choosing for yourself so I think challenges is better.

I’m planning a trip fronm Lyon (France) to Härnösand (Sweden) by hitchhiking.

I’ve got very dear friends there whom I used to visit more than onceseveral times a year.

Your sentence was correct, but this way of saying it sounds more natural. Very good use of "whom", even native speakers have trouble with it!

Fortunately, I met with some of them ion other circumstancesoccasions, but I haven’t metseen the other ones forin a very long time.

When you say you "met them" that implies that you met them for the first time. "For a long time" means for a very long duration, "in a long time" means a lot of time has passed since you have done something or something happened to you.

One of the things that put me off going there was the price of the train tickets (I'm an environmentalist, so I refuse to fly for obvious ecological reasons).

This year, I’m not richer than before so I thought that it cwould be a great adventure to travel there for free, by hitchhiking.

Your sentence is completely correct, but I think "would" sounds more natural here.

I think I could find it demoralising to mtake (go on) this kind of trip by myself, though, on the contrary, I’d find it very exciting to do that with a dear friend.

You don't "make a trip"; rather, you TAKE a trip or GO ON a trip. You use "on the contrary" when you contradict what you said in the last clause. For example, "I wouldn't find this kind of trip demoralising at all. On the contrary, I'd find it very exciting, especially with a dear friend."

I really love this guy and I feel amazingly happy to spend so much time with him, to go through these “difficulties” with him, to introduce him to my Swedish friends… It’s going to be such a great trip!

At least in American usage, and I think also in British usage, which is possibly the dialect you write in, you only need to put words in quotes if you are using the word in an unusual or surprising way, perhaps euphemistically, ironically, or copying the words of another person. Here a reader would have enough background knowledge to understand that hitchhiking can have its challenges, so I don't think the quotes are necessary here.