Build a criminal network as your BBEG
I have had success over the years in using the Organized Crime family model (an example is mentioned here) to provide scalable challenges to parties of good, neutral, and evil/chaotic alignments. The emphasis is on rivalry as the tension builder between your PCs and their nemesis.
What are the advantages of doing this?
As you may be familiar from the real world, organized crime families won't necessarily ally with other organized crime groups (drug cartels, mafia families) just because they are criminal (or if we like, for D&D purposes, just because they are Evil). They will fight with, ally with, or take over other crime organizations based on their goals, motives and opportunities.
The layered network of the criminal organization lends itself to a steadily increasing CR/challenge for your PCs with a logical "final" boss/group of pretty high CR. (Having the whole thing masterminded from the shadows by a demon, a vampire, or an evil dragon fits the D&D Swords and Sorcery genre).
- One of the best crime network campaigns I ran had a Vampire as the
real mastermind. Vampires are still a very tough boss to handle, as
are Rakshasa's. The Vampire's ability to influence and control others by fear, persuasion, and deception is built in. The Vampire doesn't care about the PCs' alignment: he or she wants to use them, manipulate them, or feed on them.
- The Rakshasa is described as working through proxies and can make
for a very hard to kill, and clever, arch nemesis regardless of
your PCs' alignment. While I have not run a campaign with the Rakshasa as the major enemy, they are custom built for that explicit purpose: misdirection and deception are their strong points. Assassin NPCs employed by the Rakshasa can be lethal if they surprise the party. (I've used the NPC Assassin on multiple occasions; nasty).
Your PCs are a criminal gang, as described.
Their goal is to take over. At least one other organized crime group is either in charge, or wants to take over. They are not intetested in being recruited just because "we are evil" but rather, they have their own goals and motives for wishing to take over the world, just as your PC's do.
As a campaign plot element, you can have the larger Criminal Organization recruit your players while they are at low levels, then as the PCs gain levels the rivalry begins to take shape. That approach is a trope that can be seen in a lot of Hollywood films, TV shows, books, and other entertainment media1.
As with various herirachical organizations, criminal and otherwise, your PCs will encounter the low level foot soldiers (guards, thugs, bandits, Knights, etc) before they run into the tougher NPC's like:
ArchMage (CR 12) with minions;
A few Champions (CR 9 warrior type, Volo's Guide to Monsters) with
squads of soldiers/archers/scouts serving them,
Allied giants, mind flayers, or other monsters who share the goals
of the crime family, or are allied for their own reasons.
A vicious cabal of halfling thieves, assassins, bards, and sorcerers
such as those encountered by Finieous Fingers and his crew.
Quantity has a quality all its own
If you go through your encounter creation steps in the DMG, or in the Basic Rules, pages 165-166, you'll notice that even low level CR NPCs (Bandits, Pirates, Thugs, Scouts, Guards, Knights) can in medium to large sized groups create deadly encounters at varying PC levels due to the number of attacks per round that your PCs are subjected to. Ranged attacks: use them!
Let's use 4 Knights as an example encounter.
4 CR 3 Knights x 700 = 2800, multiplied by 2 for being between 3-6 enemies.
For a party of 4 6th level characters, that's Deadly encounter. (5,600
XP Threshold, Basic Rules, p. 165). Granted, the encounter math is an estimate.
Tossing in an Acolyte or a Cult Fanatic to support them with spells may make the encounter a lot more difficult ...
Match your capstone encounters (or your late-tier tier 1, tier 2, and tier 3 major encounters against the party's level as they peel the layers back to expose the deeper and more dangerous enemies who are an obstacle to their long term success in taking over.
1 FWIW, Waterdeep Dragon Heist has a reasonable illustration of the criminal organization model in a published setting, but you don't need to buy that book to pull this off. The CRPG Baldur's Gate has the BBEG use economic groups and other adventuring parties to stop the protagonist's party.
If the PCs try to recruit the antagonists, a remedy for this is to role-play those NPCs into situations where they frequently and routinely betray the PCs. With a few exceptions, evil characters are typically self-serving. (thanks @R.McMillan)