Beyond Dual Band

If you aren't planning on changing cameras, I'd suggest adding anither dual band for Oiii and Sii, there are a couple on the market. With that added with you Ha/Oiii filter you could then add Sii data while getting more Oiii data at the same time. Getting an Sii only filter would only use 1/4 of you camera's sensor and be very inefficient with the 585. The 3nm filters will definitely cut more light pollution, but 3nm is very tight so you may find you like the results from you 6nm filter better anyway. 

I'm also a big fan of my Antila triband ultra rgb, it cuts quite a bit of light pollution and keeps a nice band if all three main colors. The triband does cut most yellow an brown, which probably isn't an issue in Bortle 8 anyway.

As per the previous post - getting an SII/OIII dual band filter is the best bet, as you're basically going to double down on OIII which is never a bad thing.

You can however, just get an SII filter, and essentially you'd then process your HOO image as usual from the Ha/OIII data, then take the red channel from the images shot with the SII filter, and add that into the HOO image as a yellow/golden colour… This will also work, but you'll still need to push pretty substantial Ha/OIII exposure times to get the OIII up to a suitable level, then probably need to match that again to get a decent SII signal. 

having the 2 dual band filters gives you that OIII boost, and Ha and SII are quite well matched in most places, so if you were to shoot say, 8hrs Ha/OIII and 8hrs SII/OIII, the Ha and SII will have 8hrs each - which will be plenty, and you'll have 16hrs of OIII which should be plenty for most targets to get a workable result!

I image under Bortle 6-7 skies and did find it useful to get a mono camera to partner the OSC.  The OSC is pretty good on OIII but most of the detail in most deepsky nebulae is in the HA and monocams with an HA filter achieve twice the SNR in the same imaging time.  I still prefer the OSC for broadband RGB

Here’s my story ( if you’re interested ) ……,

I image under both B8 City suburban skies in the backyard and B3 Rural skies at my Dome.
After 4 years using the 2600MC OSC with so many different dual band filters for emission nebula (ZWO to  L Extreme to Antlia ALPT 5nm )  , I decided to take the plunge and shoot Mono ( 2600MM ) after approval from the financial controller ( Wife ) It took 12 months to convince her. 
Under my B8 skies imaging emission nebula using the 2600MC OSC is no match for latest 2600MM with Antlia 3nm filters. The Ha is mind blowing and the noise floor is so much lower for similar integration time. My images have so much more fidelity and the process of creating SHO , OHS , HOO palettes is effortless and quick compared to OSC channel blending. 
These days my 2600MC OSC is only used for quick broadband targets under B8 skies as Mono is a lot of work being limited to 30sec subs in luminance and 60 sec subs in RGB.
The 2600MM is absolutely incredible under my B3 skies and Dome shooting both Broadband and Narrowband 
I can certainly vouch for jumping into Mono at some stage into the future. 
The only limiting factor is time and clear skies. I wouldn’t say I have consistent clear skies with on average approximately 4 to 8 clear nights a month. I’ve completed about 12 targets so far this calendar year using Mono. 
If your want the gold standard in Astrophotography, finance shouldn’t really be a governing factor “long term” as you’ve already shelled out significant money on your OSC rig with various filters to date. 
It’s just a question of how dedicated you are to the hobby. 

Clear Skies
Martin