Yes, you can use OEM filters. That unit isn't really an "air purifier" (they have good marketing people), it's an add-on filter holder; just a box to hold a large filter. The filters are standard sizes. The unit is sized (filter dimensions) based on the allowable pressure drop for the air handler. So you can put in a third-party filter of the same size and MERV rating as the OEM and it will behave the same.
The 4" thick filter holds a lot more filter media than a 1" filter, which allows it to go longer between changes; 1" MERV 15 filters are available, you just need to replace them more often (but your unit requires a 4" filter). If you wanted to get into a HEPA filter (what's usually thought of as an air purifier), you generally need a 4" filter for a whole-house size in order to get enough media inside for the needed air flow. Installing that TechPure unit in your air handler wasn't to make the air cleaner, it was for the convenience of not having to change the filter as often.
MERV 11 isn't really in the range that would typically be called an "air purifier". That will provide decent filtration for some of the larger common allergens. It's adequate for most people, including some of those with mild allergies. Many of the "allergy"-labeled furnace filters are MERV 11 or 12.
People with serious sensitivities, especially to allergens that are very small particles, can benefit from a filter with a MERV 15 rating. It isn't just that it filters smaller particles, it also does a better job on larger allergens. It isn't in the HEPA league, but it starts to approach it.
In a residential setting, the justification for going from MERV 15 to HEPA might be a case like a person with a suppressed immune system, who needs air as clean as possible. But most of the whole-house HEPA filters are installed for a different reason. They aren't insanely expensive, especially compared to a unit like yours with MERV 15 filters. Rather than figure out the optimum MERV rating, people just install the top-of-the-line as a simple way to ensure that the air is as clean as possible (HEPA does a better job on particles of all sizes).
Third-party filters between MERV 11 and 15 will provide a benefit somewhere in-between. The EPA has a good chart explaining some of the details of the MERV rating. Grainger has a good chart that covers the MERV ranges and typical applications. You may find that one manufacturer's MERV 14 filter is cheaper than another manufacturer's MERV 12 filter. So if you don't have special health requirements, you may be able to just go with the best available deal on a filter above MERV 11 if you want to "upgrade". Also, the difference in MERV rating is a function of the media in the filter; no other hardware, sealing, or special handling is required (it's plug & play).