Go Glass. Go combined ND / Polarized, Go Polarpro brand (not affiliated)
A good point that put my mind at ease when I thought about the extra strain on the gimbal motors issue from using the higher quality glass lens filters in lieu the plastic film ones was this: Wind resistance. The gimbal motors are designed to hold the camera stable when flying at speeds (up to 45mph max speed sport mode as stated on the DJI site - yes, not something you would do, filming at this speed but it still needs to be able to handle this) plus coping with changing the camera angle whilst doing this too ... and if built like any good specced product is built to exceed the maximum load with a considerable amount of headroom.
Put your hand out of a car window travelling at this speed and feel the force of the wind. Now consider that the P4 camera is relatively light but with that force of wind on it, the load on the gimbal motors would be quite a lot more. so if they can handle that, they can handle a little bit of extra dead weight that won't extrapolate out to being much more when flying at any speed.
Another consideration is the load of the camera weight is shared over three motors (one for each gimbal) so that considered the extra weight is manageable.
Now that I have rambled and lookiung back at that... maybe I should have just said... I fly with Glass lenses. They are heavier but I find no difference to the camera operation. glass is better than plastic film as they have a multi layered coating which reduces lens flare. I've noticed thisa when flying looking at the sun. They are easier to clean, and last longer as they do not scratch easily. They just feel like better quality.
First set I bought are Tiffen brand glass ND filters - good but the outer screw-on part of the filter is larger compared to the stock DJI ones and also larger than the more recently bought PolarPro ones (given to me by a member on here for helping him out - thank you Dave / Sharpshooter).
The PolarPro lenses look & feel like quality. They are rotatable lenses to adjust the amount of polarized light filtering - anywhere from 0% - 100% depending on the angle to the sun and the position you have the lens set at. As any of the glass lenses will not fit in the gimbal/lens protector used as support when not flying (the clear click on bracket) you simply ask them when ordering your lenses to provide you with a gimbal guard they make for this reason and they will send this out to you for free!
I don't know why they just don't do that in the first place, but it may be they only do that for members of this forum. It was only from reading another post on here about the gimbal guard issue that I heard of this, and as I was gifted the filters, I emailed them asking for one and they have sent it out - no cost. Now that is great service.