This is my experience with the Nature2 product:
I've been using Nature2 in spas for as long as I can remember (and in a pool for 5-10 years).
I would say 15-20 years but honestly I don't remember when I first found it.
3 spas (2 currently), always good results.
I can say is that I have very few problems with water when using Nature2 that aren't related to high bather load (like a all day party with heavy tub use by multiple people).
Another thing that makes me believe in it's effectiveness is that I have been using it on a hot tub at my father's house forever. This hot-tub routinely goes through extended periods (sometimes weeks) with no chemical maintenance. If there is maintenance it is usually just MPS occasionally if I'm not the one doing it, no-one adds chlorine but me.
The water is nearly always odor free and not far out of balance if at all. Since the residual chlorine is typically at 0, I figure either I'm one lucky guy or the Nature2 does a really good job. Also note that this tub can be filled or partial filled from city, well, or cross-filled from the pool so I'm pretty sure great water isn't the source.
How I manage my water using Nature2:
(to my knowledge Cense is just MPS with an aroma added, I prefer my MPS odorless)
Fill and balance water - Hardness, then PH/alkalinity, finally optionally establish low residual chlorine. Make sure to take enough time to get the hardness and alkalinity worked out. I haven't had metal issues in the spa, but you may want to test your source water one time to make sure it won't be a problem.
Add Nature2 cartridge - I mount inside my filter cartridge on my tub and in the skimmer at my father's.
Add initial dose of MPS - Usually between 1-3 tablespoons depending on when I plan on using tub.
Enter tub 15 minutes later.
Weekly I hit with 1-2 tablespoons dichlor for peace of mind sanitization. Keep an eye on cyanuric acid, too much dichlor and this can climb pretty quick. Under the conditions you mention and a method along these lines, you should easily go between fills just using dichlor and a method similar to mine.
Daily I might add a tablespoon of MPS 15 minutes before and 1-3 tablespoons after use, but I'm by no means religious about it.
Typically the only time I would add more chlorine is after heavy bather load when scuzzy bubble start to appear. I might hit with 1-2 tablespoons to start and re-check after an hour to see if bubbles still appear. I usually 1-2 punch it with MPS and don't ever add more than 4 tablespoons of chlorine total in 24 hours because it will have it smelling like a public pool for days afterwards.
In daily practice though my chlorine registers 0.0 and 0.5 starting from day 2 or 3 after the last time I used it. My water stays clean and clear with Nature2 and these levels. Note: you can't measure chlorine immediately after adding MPS as it spikes, measure before or more than 30 minutes after adding and spa has been running.
Honestly I just started regularly using chlorine in the past year or so after reading these forums. Prior to that it was only used at start-up and in rescue situations, I have relied on Nature2 + MPS for extended periods of time with very few issues.
I have noticed that for both spas there is a tendency for the PH and Alkalinity to stabilize at the low end of the spectrum around 7PH and 60-80ppm Total Alkalinity. If they look like they are getting any lower, or I'm in the mood I may add 1-2 tablespoons of alkalinity up just to get them clearly in the OK range on the test strips.
I use only test strips to gauge the chemistry and the ones I use test for chlorine, I find the MPS strips useless.
I have been pretty fortunate with this set-up and not had too many water problems for day to day normal usage.
Nature2 cartridge replaced about every 4-6 months (a bit on the long side).
I can't say there isn't a better way because I've been on Nature2 so long I don't know. I do think some people look at this system as a one stop shop for spa care and that isn't the case at all. A normal spa should be low maintenance, but it can be easy to get neurotic about it.
Also, if I got a pre-owned tub I'd give it a solid cleaning ahead of use. (1-2 day process)
Wash/scrub all surfaces including cover with a diluted chlorine or bleach (normal plain jane bleach) solution.
Rinse.
Fill.
Use a spa purge type product, wipe off any residue.
Superchlorinate for a minimum of 4 hours or longer.
Drain (polish if you want, but not required)
Fill.
Adjust chemicals.
Enjoy!
Also make sure to clean the cover and possibly apply a protectant.
The filter(s) should be new, if they aren't get new ones to install with the post cleaning fill.
A second filter(s) can be useful. That way next water change you can take your time to clean the filter, or clean it and let it dry so you can vacuum/shake out any junk that didn't come out with the hose or a brush.
