Welcome to our forum.
I would generally say that Hot Spring, Sundance, Bullfrog, Caldera, Jacuzzi, Marquis, Artesian, D1, and Arctic are all considered the premium brands. Your issue will be that today, $6,000 doesn't get you a lot of hot tub. You may struggle to find any quality hot tub that is 6-person capacity that is under $6,000. You got almost 22 years out of your Sundance, which is great and if you are looking for similar longevity today, I think you will have to increase your budget by at least 50%, assuming you are set on a 5-6 person hot tub. If you want to stay as close to that $6,000 price, then I would look into the Caldera Palatino / Hot Spring Tempo, Bullfrog X8 / X8L, Jacuzzi J275/J280 and Sundance McKinley / Ramona.