I've been slowly selling off the "collection" as well, getting out of buying new toys, & just playing a few decks I am happy with. So a couple of pointers that have been good for me on my remaining decks that I rarely get out to play:

1) Randomized but impactful effects, to guarantee you always end up playing a different game. Higher budget examples: Etali, Primal Conqueror  ^Flip, Breach the Multiverse

2) Par down your color pie. Your mono example is not for me, but it does run in-line to this idea. This is a great opportunity to sell off a couple of specific-color staples & get rid of that WUBRG land base (if you had all that).

3) Valgavoth, Harrower of Souls not really control.. But IMO groupslug, done right, can be fun for your opponent specifically because you are NOT controlling their gameplay. Though, if you wanted Rakdos control suite, you can find a few cards that will accomplish that while drawing you a card.

4) Play off opponent's topdeck will not mess with their hand/limit resources. If you make a deck that does this, you have less need to "stay caught up/enfranchised" because you are as "caught up" to MtG as your opponents' are with this deck. This will be impactful, but always play differently (good qualities if you play infrequently). Eg. Rakdos, the Muscle

That also raises an interesting point as well: the meta at-large can influence the idea of "toxicity". So, if the prevailing deck strategy of the time is "control wins" - as it was for quite a time. Well, therefore, a majority of anyone sitting across from you are playing the hard control route, then you are constantly strategically "swimming against the tide" with trying to play a damage-clock game someone is eventually going to Approach or pillow-win or something even if your deck stacks up well against that playstyle, it usually won't stack up well against 3 other decks trying to accomplish the same thing for themselves.

However, if you have someone else playing a similar strategy at your table, suddenly the strategy balance shifts so that you will not be the odd-man out standing in 75% of your opponent's way... This meta diversity balance to me, shifts how toxic a given card will be. I've played games where somehow everyone was just playing midrange pain & basically a player put "tithe" & "rhystic" out, but because we were playing pain-grind that player was out 2 cards with no benefit forever (usually it should have been a somewhat "toxic" opening hand, but the unusual meta composition had nullified the toxicity)

Fortunately, the game has branched out quite a bit these days in regards to viable archetypes, so I see less 2x Oloro, 1x Narset games than I used to.

Depends on strategy, so it ends up being subjective. I purposefully make a large amount of "midrange" - "midgame" decks that are pretty adaptable & I like playing a slug/death by 1,000 cuts style (cards making a gameclock). As such, the whole "what's your wincon?" question has that unconventional answer of: "almost 50% of my deck is a wincon" - but this is due to life & resource drain. So, one of my boogeyman is combo-centric pieces. Where I build a lot for some back-&-forth fencing, those "oops all combos" decks do the opposite. It drives me nuts when we have >3 people in a hotly contested game that is moving along well & then some zero-panache combo just drops down seemingly from someone playing a different game & I wonder why I wasted my time.

So, where other people see combos as fair game, from my point of view, with my prevailing strategy, I will typically view them as toxic. They are significantly less "toxic" to me if each combo piece stands well on its' own 2 legs within the deck without another piece (so if the combo pieces have amazing deck synergy on their own).