Round, screw-in, filters are what you need for highly attenuating ND filters.
Tiffen have a very interesting variable (2 to 8 stop) silter that looks very interesting - https://www.adorama.com/tf77vnd.html?kbid=12417
. It solves a lot of problems that you can run into with a fixed value ND filter (sometimes not enough attenuation, sometimes too much!). Hoya do a similar filter (1.5 to 9 stops) - https://www.adorama.com/hy77vnd.html
I've been happy with Hoya filters myself. I have a 400x ND Hoya (9 stops, but no longer in production) that I use for this type of long exposure. Tiffen makes a relatively inexpensive 10 stop filters (https://www.adorama.com/tf77nd30.html?kbid=12417
). They also have one which also blocks IR light (https://www.adorama.com/tfw77irnd30.html?kbid=12417
). The IR light that leaks through some ND filters could, I guess, have an effect if the camera didn't have a strong IR blocking filter in fornt of the sensor and the exposure time was long. Most people don't worry about IR Transmission. I've not found it to be an issue for me.
The Breakthrough filters are very good (https://www.adorama.com/btx3nd677.html?kbid=12417
), but more expensive then Hoya/Tiffem and fixed in value.
Whether a 3 stop and 6 stop filter pair is better than a 10 stop filter probably depends on what conditions you shoot in. You can change exposure time using aperture and ISO setting, as well as stacking a polarizer (which you probably have). I don't know what prices are in Goa, but the 10 stop Tiffen XLE 77mm filter is only $40 here in the US (https://www.adorama.com/tfw77irnd30.html?kbid=12417
). I mighty start out with that one and see how things go.
One thing to watch with the variable filters is that they since they consist of two polarizers (usually one linear, one circular), they can produce unwanted polarization effects (such as banding) if the subject is generating polarized light (e.g the sky on a clear day). The effect gets worse with wideangle lenses and high density settings.