Should i level a rider?

As long as you have a good st and AoE np rider you don't have to level any other unless you want to. I do prefer Santa np5 over iskandar

I’d say if you want to level up other riders it’s fine but since you already have both a st and aoe riders leveled up its not necessary.

Well from what I can see I think it's better to level up all of them but if you need to choose it's better choose Ozzy and modred due Ozzy is good boss killer and more tanky and versatile than iskandar and modred is a good np spamers (way better than Ozzy and iskandar in terms spamming No)....also I advised you to not neglect Martha, her attack is decent but her skills definitely useful

Surfer Mordred is kind of unique, if Ozy finish his problem using bruteforce then Surfer girl use Tsunami to drown her enemies ( just imagine if you were getting hit by multiple Tsunami waves ).

Arts have their own ways to clear the game contents, Arts servants have the most unique effects in their skills and NPs and they're not as straight forward as Buster teams.
While mostly Buster servants is pretty straight forward.

You want Nuke and Burst ala Buster, sure Arts have it too. Stall and unkillable servants, they're the Grand master of this.
Making your enemies stunned or disabled the entire time, Art is the master of this.
Spamm NP, it's the basic for Art teams.
Debuffing your enemies till they can't do anything, Art can do this.
Breakbars is not a problem for Art teams, Arts team gimmicks and playstyle is more complex than breakbars.
There are more, you'll never encounter the same Art playstyle everytime you change your Art Teams.
Here's some team comps if you'd like to try it :

For Mordred NP Spam, Mordred + Nero Bride + Tamamo.
Squirtoria debuff NP spam, Squirtoria + Tamamo + Asterios.
Mainstream Art NP Spam, Vlad III + Tamamo + Waver.
Robin Hood Nuclear Warhead, Grailed Robin + Tamamo/Caesar + Waver.
Li Shuwen OHKO ALA Scathach, Li Shuwen + Tamamo/Waver + Mozart.
Double Asterios + any art servants vs multiple hit enemies.
Hans/Mashu + Hans + Mashu is an art stall zombie team.
Joan of Arc + Hans + Mashu, just another Zombie team.
Marie Caster + Marie Rider + Marie Caster, Teenage whassup homies turtle.
Saberlot + Hans/Tamamo + Waver/Hans, Art Spam + Crit team.

Mozart is likely used for 1 turn kill or short fight.
Paracelsus is like a 3 star Caster Gilgamesh with longer skill CD.
Asterios for more agressive and to cripple focused enemy.

For Raid/Challenge Art team alike, Squirtoria/Robin Hood + Tamamo + Georgios/Mashu + Waver + Asterios/D'eon/Helena/Mozart + OG Nero.
Battle Suit for more buffing/debuffing/Harder fight, Memories of the Lunar Sea for Art Burst.
For more consistent NP spam, don't use all of those Art buff in a single turn, try to divide it so you can consistently spam your NPs.
For Art Burst or nuke, it's a different case.

From your rosters, i think it's more than enough. Maybe you can cover some ''hole'' in your Chaldea.
Because Ozy himself is a great rider, and you also have Iskandar as your wave clearing servant.
Martha is also useful for harder content and challenges alike.
But if you still want to try for Art teams, then go ahead. It's up to you.
No matter what kind team you use, be it Buster, Quick or Art team you'll play all of it eventually if you play for long enough.

Level her too. I don't understand that "I have many of their class". At the end, most servants are worth of raising, and if you like them even more worthy.
Ofc, at the beginning we have to make priorities, but when your rooster is pretty done, you can start leveling others too.

Unless I dislike them as a character or their design, I level every servant I get, though that is the completionist in me. Every servant has a different skill set and a different application, even if they are similar, they are good at something the other might not be so I tend to raise them all for any challenges where they may be needed. I would prioritise one single and one AOE first, you can take the time with others if you choose to do them all.

To show a different point of view, i do understand the logic behind raising only the most efficient/favorite servants. Raising servants takes too many mats, extremely crazy amounts of QP...basically, lots of time wasted grinding to develop an unit that you don't need and you might use (maybe) against the extremely specific enemy type they counter (unless they are something extremely awesome like Gilgamesh).

I already have Artoria, Siegfried and NP5 Saberlot...Why would i raise Brave Liz? That would just a waste of my time for an inferior option that i don't even like.

I have SurMo. Pro: highly spamable NP. If you manage to get her an NPAA brave chain and use her Arts buff skill, she'll potentially be able to use it in back-to-back turns. Con: NP needs to be spamable because it doesn't hit very hard.