You've got plenty of ways to lay on the -1/-1 counters, but you need some hard removal to take of bigger stuff. Terminate should be included.

Rakdos Charm is a versatile tool that is highly effective against multiple types of decks (artifact, graveyard, tokens).

Vandalblast is super great. Use it turn 2 to counter a Sol Ring, or blow up all the mana rocks later.

Hateflayer is a powerful beater with synergy with your theme.

Dreadbore is another hard removal spell worth considering, especially since -1/-1 counters don't work against 'walkers.

Talisman of Indulgence should probably be included as another mana rock.

Some permanents (enchantments) are near-impossible for Rakdos colors to deal with, which is why Chaos Warp continues to be a valuable removal too.

Those are all great cards for the deck, but I'm building this as on theme as possible for casual play and none of these put -1/-1 counters on creatures. Well, save for Hateflayer, but I'm just not a fan of a 7mana creature with no immediate value.

I will expand the maybeboard tho for people who want a stronger version. Thanks

I know you said you're not a fan of high cost creatures, but thoughts on Grim Poppetor Lockjaw Snapper? I have the feeling lockjaw would be a way to weaken bigger dudes, and if you just chump him into said big dude you could cripple it. and Grim poppet can just keep distributing -1/-1 counters so long as you can keep putting them on him, which I would assume would be through effects like "Give all creatures -1/-1 counters

Serrated Arrows might be faster at 4 CMC and better protected than Triskelion (doesn't die to creature removal/damage/board wipes) and combos better with your other peices, notably your commander. You just have to regenerate it with proliferate one time more often. While damage heals, you might often find uses for having -1/-1 counters on opponents' things. Thought it might be worth mentioning.

That is a very good list for Infect, there are a lot of cards with Wither that can really beef it up as well. It does look like you have considered some of those already. Might I suggest Sudden Spoiling as a great way to stop a stack as well as make an attack on you, or block very hard to deal with. It has worked to great effect for me.