Reigning In A Homebrew Obsessed Player?

Hey all, need some advice on a player who is constantly trying to step outside of the rules of the game at my table.

Context wise; I'm great friends with everyone at my table and my players and I have been going strong for nearly 10 years consistently. This is not some DND Rando I play with, and it's not someone new to the game or new to our table.

Core Issue; This player has always been out there and is constantly coming up with far fetched ideas and characters- Some of which I let ride out, and some of which I have to shut down. A great example is when they wanted to roll a LVL 1 Cleric for a new campaign, but this cleric was incredibly elderly, a war vet, and was once a LVL 20 heroic adventurer- They are now at the power of a LVL 1 Cleric because they are old, retired, and possessed by a demon that sap their holy energy- Making them literally half-angel and half-demon. I thought that was simply too off-brand and complicated for the game we were set to run, and said no.

Current Issue; The player is playing an inventor of sorts, and while I want to encourage their creativity they are constantly asking to step outside the rules of the game and make items and tools that I have to spend personal time homebrewing rules for. I recently made them a 5 page document filled with alchemy ingredients, their effects, and homebrew ammunition types because they want to be able to "Shoot utility arrows like in Baldurs Gate 3". Beyond that, they are constantly asking to use Homebrew spells or infusions, asking to swap the damage types of their summons and spells, and asking about homebrew items.

Normally, ofc, the answer is "Just talk to your player-" but I promise I already have on multiple occasions. I have cautioned, warned, and asked politely to please try and just stick to the core set of books- PHB/DMG/Xanathar/Tasha/Etc but it's never enough it seems.

Frankly the worst part about the issue is that when I DO say "No" to an idea, I get a weird amount of pushback. I explain my reasoning "That's not really in the rules, I'd have to make something up for that, Fireball isn't designed to do Cold and Fire damage-" and they go "Yeah but my character uses Elven magic and in lore Elven magic is blah blah blah-" or "My character could simply change the arcane runes they use to blah blah-" and it's like... I said no, I asked you not to do this every session, and it's still happening.

Any advice on what to do?