I know it's been a while since you posted this and I don't have anything specifc but, um. When I try to make an evolution method for a pokemon (fakemon or otherwise) I try to think of the in-universe lore and stuff going on.
Like, this drider evolution is of the base regular Ariados right?
But since it's not in 'mainline Pokemon', in-universe it can't be an 'easy to reach' evolution method like Level Up or Max Friendship without some other condition or else it would've been seen/more common and thus it must be something rarer like a new Item or move, some combo of things that people prior to now didn't try (like idk Level up at night while holding Escape Rope or something) or perhaps it can be by a more normal evolution method but justified in-universe by an environmental factor unique to your game's region, like maybe the Ariados here prey on/interact with other Pokemon unique to the region they need to evolve/be healthier? Or they're sensitive to a weird phenomenon like Hisui's space-time distortion or the aftereffects of a legendary where they live or something.
Another factor that might help to consider is how common in this region this drider evolution is: Is it common enough the player will get info from NPCs in universe on how to achieve (aka a more 'common' evolution method), or one you'd like to be a mystery and/or difficult enough to do that it 'tests' the player's worthiness for such a powerful pokemon?
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For an example of how lore can help decide evolution methods: just recently designed a regional variant of Purrloin/Liepard where the 'plague cat' Purrloin is basically considered the region's Magikarp (weak, overly common and a poisonous pest no one wants around) but it's evolution is a highly prized and very rare pseudolegendary that people don't know how to obtain aside from running into one in the wild and MAYBE catching it, so the evolution method has to be difficult to achieve... But it also has to be a method this region's Purrloin can achieve in the wild since there are in fact wild Liepard running about, even if they're very very rare and their catch rate is abysmal.
Cue me thinking about what kind of routes the two can be found in and realizing 'oh, if the poison Purrloin are everywhere until driven away but the Liepard are only found on highly isolated and dangerous snowy mountain peaks covered in storms then good odds an ice themed evolution method is the way to go' but then its like, do I really want to make this evolution purely limited to a location (via the Ice Rock/snowy mountain under snow storm conditions) which is likely where these Liepard can be encountered in wild already and thus inadvertently 'justify' not catching and putting in the hard work of raising the weak Purrloin all this time?
Or do I instead go with something obtainable much earlier such as having them reach a high level (45? 50?) with max friendship while holding something Ice themed (like an Ice Stone, Icy Rock or maybe an Ice Plate even) to instead reward the player's loyalty to their stinky little cat buddy, while also making it clear why in-universe the evolution method isn't known?
Alternatively I could ditch the 'ice' theming to instead lean harder on the 'Purrloin endures and overcomes great hardship to become a famed dragon killer' angle by having it evolve via White Stripe Basculin's 'it must survive X amount of hit points lost without being KOed or healed at a PC' method or perhaps a 'it must KO [large number] of pokemon without fainting' method, which would force the player to really pamper their Purrloin but rewards them for their efforts with a VERY powerful pokemon far earlier than it would normally be obtainable~
(Would probably have to add a minimum level of 40 or so for these methods though since getting a pseudolegendary by accident just EV training at the start of the game in route 1 would be more than a little broken and kinda weakens the whole 'Purrloin really struggle to earn their evolution' vibe of things XDDD)
...Still haven't actually made up my mind on what my Purrloin's actual evolution method IS yet but hopefully seeing my thought process helps gets some ideas for your drider evolution going <3
