Nobody is obliged to give you money towards your wedding, unfortunately.

I don't know your relationship with them but I would say sit down with your parents and explain that you feel pressured to pay for an extravagant wedding to please them and it's stressing you out. The results of this conversations could be: them reassuring you that they don't care what your wedding is like as long as you're happy, you basically warning them to keep their expectations low so they're not disappointed if your wedding doesn't live up to their expectations, or they might offer you more money.

I don't think you have to be religious to use a church banquet hall - unless you're against religion in some way this could still be a cheap option.

You could also get married in a park...the fees for the public rental or licensing fee which allows you to use public land is usually pretty cheap

you could get married at city hall like a week/day/year before your wedding and have one of your friends "marry" you at your wedding to save money on an officiant (they usually cost more to come to you)

I've been to weddings with fake flowers, brooch flower bouquets, and another which used real potted plants and tied on fake flowers to make the plants more interesting.

You could skip flowers all together and go for a more interesting venue like an aquarium or botanical garden let nature be your "florist"

I wouldn't advise doing a potluck dinner thing...that would be the place where I would spend my money on a caterer but you can also go cheap and just do cocktails and hors d'oeuvres, or an afternoon tea style wedding, or just tapas.

I've been to a couple weddings in Norway and I like how they did dessert. The couple had a small wedding cake (could be professionally made or just something small and cute with a topper made by a friend) but then they also asked several other guests to bring cake. Now, I'm totally biased because I'm a cake lover but it was so awesome to have 10 different flavour cakes to choose from...you could also mix it up and ask someone to bring doughnuts, someone else to bring candy, someone else to bring pie...

I say skip the favours! I didn't even know they were still a think until recently...I went to two weddings one where one of the guests made these cute bracelets for all the female guests as her wedding gift to the bride and that was the only favour; and another where the groom's sister has downs syndrome and her adult activity group made a bunch of super cute notebooks.

An alternative wedding favour could be posting a sign at the reception saying something like "in lieu of wedding favours a donation has been made in your name to [charity]" Nobody has to know what you spent or be stuck with some dinky thing because you felt obliged to have favours but didn't have the budget.

I would say just budget with exactly what you have now (-25%) and don't plan on having any other contributions. If you go a bit over budget (you will), and if you get any other contributions (you might not) then that will all be a bonus.