Some people have already given you great tips. You can find even more advice on youtube and other places.
If you are in an FC you manage, submarines are a good way of making gil passively and without MB interference once you set it up. Expect the setup to take at least about 6 months, but once it's working at full capacity, you only have to pop into the workshop every other day to press a few buttons and retrieve your money :P people love to look down on others for telling you this advice, but if it's an option to you then it'd be silly not to use it. If you want to make even more money, go for housing exteriors. But that takes MUCH more work than going the jewelry route.
When the unreal market was doing very well back before DT, my friends and I made a collective 350M just by selling the stuff you can buy with the faux leaves and supplementing it with submarine management. We made that amount because we wanted an FC mansion (which we got, because Phantom is a ghost town and no one bid for our mansion except us lol)
I don't do any crafting or gathering, but doing some market research and staying on top of trends can net you a lot of gil. If you want to make insane amounts of gil and do it quickly, it'll take more work. If you don't mind building your way up there slowly, I bought my 20M medium just from roulette money. Make sure you always go as adventurer in need if you can help it, since it'll give you a little extra. Desynth all of the gear and drops you find and sell the raw materials. If you do extremes, desynth the weapons since you can get lucky and sell stuff like the Hydaelyn crystals that got much more expensive than they used to be when the glowy weapons released. Believe it or not, since Variant is dead content, anything that comes from that can get expensive simply because there aren't many people who do it anymore. You can solo it easily.
I exclusively do battle content, so my advice is limited to that. I'm not saying the players who are poor are because they want to, but some of you need to stop getting scammed buying glam off the MB lol GamerEscape, GarlandTools and crafting your own stuff is your best friend. You can still get everything you want for much cheaper if you aren't completely against inconveniencing yourself to get it crafted. If you can't craft it, always check Universalis to find the cheapest price and get that.
You can also save up on teleports by buying Aetheryte Tickets from doing hunts or getting a -15% teleport fee buff scroll from telling your squadron to go do stuff once a week. That's the equivalent of a Rank 3 FC buff, with the caveat that it isn't a 24hr buff.
There's also another thing to consider: XIV had much more expensive housing in the past, and it was meant to be used as a way to entice Legacy players to spend their 1.0 fortune and hopefully put them closer to an equal level with the 2.0 gang. It was a long time ago, but I feel like it's worth mentioning because that is everything housing is; a gil sink and a status symbol. Whether we think decorating anything bigger than an apartment is locked behind this is unfair is another topic, but housing was always designed to be a gil sink for those who can afford it. The good thing is that making gil has become leagues better than it used to be (and housing has become much cheaper since back then, go check out the original prices if you want a jumpscare) from what I've heard.
I hope you find a comfortable way to make the gil you need. It might be a good idea to let us know what kind of content you usually do, so that people who are familiar with it can give you more tailored advice.