I have been frequenting a thai restaurant (in the USA, not Thailand) and I have been exchanging a few sentences with some of the thai speakers that work there.

When I greet them, I am saying สวัสดีครับ คุณสบายดีไหมครับ. However, I would like to be more casual. I have heard them say หวัดดี เป็นยังไง and I have never heard them say ข้าง on the end either, they just say เป็นยังไง.

I just want some more options.

I have been frequenting a thai restaurant (in the USA, not Thailand) and I have been exchanging a few sentences with some of the thai speakers that work there.

When I greet them, I am saying สวัสดีครับ คุณสบายดีไหมครับ. However, I would like to be more casual. I have heard them say หวัดดี เป็นยังไง and I have never heard them say ข้าง on the end either, they just say เป็นยังไง.

I just want some more options.

เป็นยังไง is much less formal, it is OK to use for close friends.

ขอบคุณครับ I am still looking for a few more suggestions or opinions on this matter.

ยังอยู่ไหม คุณมีดิสห์

I really do not have much to add except to reiterate JCCR's advice that it depends on how familiar you are with each other.

What you have heard them say are normal greeting phrases, but less formal than สวัสดีครับ คุณสบายดีไหมครับ . If you are around the same age as them and have gotten to know them it would not be wrong to use the more familiar terms, but as a rule Thais take longer to get familiar with strangers, so it is hard to give advice for each individual case.

If you are unsure of where you stand with them, just tag on a ครับ at the end. It's a good idea to get familiar with the kinship pronouns too; learning how to relate to people based on your relative age difference. The casual greetings go a lot better together with พี่ and น้อง than with คุณ ... but if you still have not gotten to know these people and have only exchanged a few phrases, you are probably better off continuing on the polite level.