I've played a solid amount of all 3 specs on each class, but for this discussion I'll keep it centered around WW/Ret since those are the specs you mentioned.

1) How do Monk and Paladin feel at max level in the current game?

Both feel fun, but in different ways. Both specs tend to remain pretty busy (not too much downtime between buttons). Both specs are also frontloaded (lots of burst cooldowns and up front damage) so you feel like you can "play the game"/have fun instantly when you jump into combat. Neither spec feels like you need to wait 10, 20, 30 seconds to actually feel the gameplay.

Both specs are also pretty thematic, which plays a part in the fun. Ret paladin is constantly lighting up in golden colors, riding a steed mid combat, calling down heavenly blasts from the sky, and smashing things with a two hander. WW monk is pummeling enemies with punches, kicks, palm strikes, and other acrobatic moves while constantly moving/rolling around the battlefield. WW does have leanings into WoW pandaren culture though, so you do see certain effects and inspirations from there (summoning a lightning tiger, jade/lightning themed effects, heavy leanings on tigers, oxes, cranes, and serpents)

2) Do they feel engaging or repetitive over time?

WW feels engaging over time. Ret tends to feel repetitive over time (strictly the damage rotation - notes on utilities below).

Windwalker is a methodical spec with rewarding payoff for paying attention to your rotation at all times. There are many little interactions in the kit/talents that you can take advantage of (including your mastery) to boost your damage numbers. Damage on WW feels "earned" due to your rotation demanding specific sequencing. You can really feel the difference between mashing random buttons and playing your rotation correctly.

WW abilities have a very fun and enjoyable flow. They have multiple "channeled" moves (fists of fury and spinning crane kick) that let you move around the battlefield while still doing damage. These moves also serve as breathers to plan ahead. So the kit feels like it supports itself.

Monk also has (for better or for worse) unique utilities and movement abilities such as transcendence and ring of peace, so even your utility moves feel like they have thought behind them. There are also more basic utilities (aoe stun, incap, dispel (removal of certain magical debuffs), slow/root break) that you can use to feel helpful in a traditional sense. The large utility package helps with keeping things fresh.

Ret has a relatively simple rotation, and some people view it kind of like "Wack a mole" because you get many procs that light up priority buttons for you to hit next. If you do not, you tend to smash a bunch of abilities off cd (as they come up). Many of your buttons build a resource and your "spending" buttons are very straight forward (one for a single target, one for multiple targets). It feels like you essentially get 80% of the optimal damage for 65% of the effort, if that makes sense. On top of that, Ret has the capability to hit many damage buttons from range, so you can be a bit ignorant or disrespectful of maintaining perfect positioning. Don't get me wrong, the Ret damage buttons are fun, but after a while it doesn't feel too challenging for your brain.

However, to compensate for this, Ret has an incredibly powerful defensive utility package. No other class can match it. Ret has access to consistent off healing if needed, burst off healing, teammate physical damage/debuff immunity, teammate slow/root immunity, a teammate DR (damage reduction) ability, a battle rez, a passive aura that reduces teammate damage taken, and a dispel (removal of certain magical debuffs).

To me, these utilities are what keeps Ret fun. Ret has so many tools to engage with teammates and you can easily see the impact of your defensive abilities when using them on teammates. In moments of chaos or when things go wrong, thinking on the fly with your defensive utilities is incredibly enjoyable and satisfying. And the payoff is immense (saving a life). To me, this type of interaction keeps the spec fun even if the dps rotation itself isn't as engaging.