FTM vs CTM

I feel like fast tm's will get more valuable, ive got 20 and thats after all the beldum, dratini and venasaur I evolved last weekend

Being able to retool a pokemon's moveset without needing to power up a whole new one will make fast tm's in my oppinion a lot more valuable. For instance if I want a weavile, like you said, I wouldn't need to sink the shear number of candy and dust and charge TMs in to a second one to get one for both sets, I'd only need the one plus the candy and dust for the extra charge move (maybe taking 1 TCM as most pokemon have 3 charge moves so you're guaranteed a max CTM need of 1 most all of the time) and then the single fast TM every so often to switch it to countering what I need.

Certain pokemon wont really benefit from this, things like tyranitar, dragonite, gengar, maybe metagross, who have really onely one best attacking set that's not legacy locked with a fast move (metagross' best set is legacy locked, but it has no best fast move that's legacy locked). So I dont think One will become more valuable over all, but one tm might hold more value to one play than it does to another depending on their set up.

I don't think the value will so much as flip.

If we're evaluating their values right now, setting Charge TMs to a value of 100 as a reference point, I would say that currently fast TMs are worth about 10.

After the second charge move, I would say charge TMs go to about 40, and fast TMs go to about 30.

Why?

Charge TMs will still have value for lots of pokemon. There are plenty of pokemon with no secondary typed charge move. For example, Raikou. You're TMing for Wild Charge. That's it. You're not going to spend dust and candy to unlock two electric charge moves. Also for type-different sets that aren't useful in PvP - e.g. Machamp. You want Dynamic Punch. Unless the dust cost is low enough that it's worth saving you the risk of wasting charge TMs, there's no reason to get Heavy Slam on Machamp. He won't be useful on PvP or gym defense, and on offense you'll always use Dynamic Punch.

Fast TMs go up in value because, as you mentioned, you can swap moves for a single pokemon. However, you don't need to swap all that often. Like in your example with Weavile - how often do you need double dark or double ice weavile? Likely only when a new raid boss shows up, and it sticks around for a month. Even a full team of them would only work out to a single fast TM per 5 days - assuming raid bosses alternate between your two types (any bosses in between that you simply don't use the pokemon for, is even more idle time without needing to use fast TMs on it). It's unlikely to be a situation like where SB Mewtwo swaps between PC and C so it has the former for psychic raid boss and the latter for Machamp raids all at the same time - which exists in the current case and even then you don't go through too many fast TMs.