For clarification, I have searched up the forum beforehand to see if there were any similar questions and there wasn't. What I want to know here is what is the best onsite profession to invest in (e.g expanding cooking/mining/dens) since I'm looking more to long term levelling and playing. As a new ftp fellow, cosmetics and gene related items are definitely off the table atm.

If you have any tips feel free to let me know :D

Mining is the only expansion I've bought personally. I might expand the cooking pot later, but once you hit the second mining area, it becomes practically 50/50 whether you have enough ores to smelt to make three bars for the activity, and then you have to go back and can't finish the mine 15 ore activity... I might expand again now that I've hit the third area because the problem only grows, but 50ms is pretty dear, you know? >.<

Seconding this--the later areas have so many different ore types that it's hard to get five of a kind without expanding or constantly dropping ore.

While a larger cooking pot is nice you can just check cooking more frequently. You can cancel a pot, add a few things, then let it continue to basically never let it run out, and there's no stamina penalties like with mining.

EDIT: I see you also mentioned dens--I like the extra dens, they're easier for sorting, and depending on how many wolves you have it can be cheaper buy a new den than expand an existing one. There isn't any explicit "gain" from spending ms on these though, so it's really down to how much you can afford/what layout you value.

I would get the first mining expansion for quality of life if nothing else. It's incredibly frustrating to not be able to smelt three bars just because you have one extra ore of something. Unless you are very patient, I would do this just to keep yourself from losing interest.
That being said, I have never been able to make much money off mining--maybe others have different experience. ATM there is kind of a demand for copper bars to give to the rockbreaker ox, so you might have luck selling those.

Easiest way to make MS is to make lots of pebbles and then sell those for MS. The usual wisdom on that is to grind the 4th level bosses, but you go through a lot of food doing that. An alternative would be to sell food to the people doing heavy grinding. This is what I mostly do--heavy food production (though farming, cooking, and fishing) and alternating boss grinding vs lower level grinding.

You also make a surprising amount of pebbles just from following the activity buttons around. Don't forget to play with your companions as well.

I am quite patient and it should not be particularly frustrating for me as long as I don't put too much attention on it, but compared to the other options it seems the most wise. I have thoroughly considered everyone's response and I think I'll be saving up for that one mining expansion! Thank y'all for your input ^^