This read like D&D (bad) vs. Torchbearer (good) argument.
Every pnp RPG that requires a DM is based on DM-fiat, if it doesn't then it doesn't require a DM at all and the DM should be playing as a player. For a DM it starts with basic things like what kind of adventure are we playing, even if it's published adventures, the DM chooses which one, etc.
It also seems the OP doesn't know a whole lot about D&D, the Alarm spell comes from a time when random encounters were codified in the D&D system. You roll every x hours, depending on environment, day/night cyle and possibly alert status. If you roll Y, you roll on table Z for the actual encounter. Often with a random number of enemies stated in the random encounter.
Essentially what Torchbearer does, is emulate that old skool style of codified gameplay with another system, while D&D 5e has become more freeform from it's early editions. BUT the new DMG 2024 still has random encounter tables, just not everyone uses them anymore. But forgetting about them all together, especially when in relation to the Alarm spell... Kinda of a big oversight on the OPs part...
There are extremes in DMing, from those that follow strictly all the rules, no exceptions and if there are no rules to follow they seem kind of lost. The tyranical DM where things go exactly as the DM wishes and only as the DM wishes or the DM the conforms to all the wishes the players have. None of these extremes generally make for a good game, what's a good game depends on the people playing. When you've played pnp RPGs with your specific group for decades you tend to figure out what each one likes or not. BUT... I've been known to strap players to the hood of a metaphorical car and drive over any lines, Mad Max style, to see where that line is for that particular subject within the group.
grins evily I've done that with blood, guts and body horror. Where one of the most composed players in my group pretty much said "Nope! This is WAY too much! Please stop!". But please don't do this with people that aren't your friends (and you've known for a decade+ )...
If you have a problem with the fiat your DM shows, that is an issue with trust or trust misuse, or just poor communication. Imho there is no perfect way to solve this. Because every group dynamic is different, different people expecting different things, having different relationships with each other. A group of friend playing D&D is a whole different beast from a group of random strangers playing the same game with the same DM...