Horwath
Legend
100% this,A character's game mechanics that are given to a "base class" versus given to a "subclass" are arbitrary. The designers just select a handful and then flavor them with their cutesy names to say that "these mechanics are from being a Cleric, and those mechanics are from being in the 'Light domain'." But what those mechanics are is just whatever a designer came up with. So there is absolutely no reason why a player or DM themselves couldn't just arbitrarily say that certain mechanics one gets at 1st level from the "class" are now going to be mechanics they get from their "subclass" and just assign them that way.
If a player is selecting the "Light Domain" at 3rd level because they are a Cleric of Lathander... then they can just say that certain mechanics they are getting at 1st level are because of their worship of the "Light domain" rather than because being a "Cleric" gave it to them. Just arbitrarily decide that the Light and Sacred Flame cantrips you select at 1st level are due to being a "Light domain" cleric. You're a Cleric of Lathander... there's no reason why that god granting you those two cantrips wouldn't be due to His domain. Lathander's "favored weapon" is the Mace... so there's no reason to not to believe that the reason why your Cleric is proficient in the use of the Mace is because of their training as a Light domain cleric of Lathander-- so their Simple weapon proficiency is attributed to the domain as well. Just take a couple of your 1st level Cleric features and "decide" they are now "Light domain subclass features" because why not? It literally does not matter.
I mean after all... the game designers have already said that when a character of 1st level sees their proficient Skill list in its entirety... some of those skills are "from their class" and some are "from their background". But once the selection has been made and you now have your skill proficiency list, where those skills "come from" never matters. You just have your skill list. So by the same token, once you have your features it doesn't matter where those features come from or what arbitrary bucket they came out of... so just reassign those features at 1st level to the various buckets as you want-- Class, Subclass, Background or whatever. It doesn't matter in the slightest. You have your features and you are going to use them, and you're never going to ever think about where you got them after the fact.
subclasses are just feat chains.