These are two separate questions. Both read like an X→Y problem where you're asking for help with problems encountered using your perceived solution to a different root problem. It might be better to ask how to accomplish what the basic goal is for using your attachment in the first place.
Are there any RF lenses with similar lens attachment points that don't disrupt focus/zoom? (Ideally similar focal length.)
Looking at the RF lens lineup: The shortest focal length prime lens that does not extend the filter threads when the lens is focused at closer subjects is the RF 50mm f/1.2 L USM. The RF 85mm f/1.2 L USM and RF 100mm f/2.8 L IS Macro USM also have this characteristic. The soon to be released RF 135mm f/1.8 L IS USM also appears to have the same type of fixed filter thread design.
Please note that lenses with similar focal lengths, such as the RF 50mm f/1.8 STM and RF 85mm f/2 Macro IS USM, do not have the same type of design and the filter threads do move forward as the lens is focused closer. However, the lens hood connectors on those lenses remain stationary as the inner barrel extends inside of the lens hood connector rings.
All of the current RF zooms lens lineup have lens barrels that extend as they are zoomed to longer focal lengths.
Will switching from EF adapter + EF lens to RF lens impact my crop / fov? Say at 28mm how similar will the fov be for an object near the front of the lens?
Using an EF to RF adapter simply spaces the flange of an EF lens at 44mm away from the RF camera's sensor when attached to the RF camera's flange ring that is 20mm away from the sensor. The image circle projected by an EF lens attached to an EF camera is exactly the same size as the image circle projected by an EF lens attached to an RF camera using an EF → RF adapter.
In both cases the lens' flange is 44mm in front of the camera's sensor (or film) plane.
- With an EF camera, the camera's mount ring is 44mm in front of the image plane.
- With an RF camera, the adapter's lens-side mount ring is 44mm in front of the image plane.
If the same EF lens design were to be used in an RF lens, the lens barrel would be lengthened by 24mm so that the flange is 24mm further back from the optical elements in the RF lens than in the EF lens. Thus the lens elements of the EF lens and the RF lens would be the same distance from the sensors of their respective cameras. Only the point at which the lens ends and the camera begins will have changed.
If a 28mm EF lens + EF → RF adapter and a 28mm RF lens on a full frame RF camera were both focused at infinity they would provide very similar Angles of View (AoV), within the accuracy of their actual focal lengths versus marketed focal lengths. Some "28mm" lenses may actually be 31mm or even 32mm lenses. Other "28mm" lenses may have actual focal lengths of down to 26mm. The extremes of the range tend to show up more on zoom lenses, but it wouldn't be that exceptional for a "28mm" prime lens to have an actual focal length, when focused at infinity, of anywhere from 26.5mm to 30mm.
As the focus distance is decreased from far to near subjects, however, different lens designs with different optical formulae can behave differently with regard to AoV and magnification. Typically, a simple lens with unit focusing will exhibit focus breathing that causes the angle of view to become narrower as the subject distance is decreased. But this is far from universal, especially we consider that most mass marketed lens designs are complex compound lenses, many with internal focusing elements rather than unit focusing. One version of Nikon's 70-200mm f/2.8 was notorious for increasing the AoV to the equivalent of about 140mm when the zoom remained set at 200mm and the lens was focused at its Minimum Focus Distance (MFD). Very expensive and complex "cinema" lenses can be designed to demonstrate almost no focus breathing when the subject distance is reduced.
Two different 28mm lens designs, even ones sharing the same mount, can have different MFD. This affects the size of the same object due to the difference in the minimum allowable distance of that object from the camera. Remember, focus distances are always measured from the image plane at the film or sensor, not from the front of the lens.
In short, when considering two lenses of the same focal length used at close focusing distances, both focus breathing and the Minimum Focus Distance of each lens design must be considered to be able to predict the difference in the size of the subject as projected onto the camera's sensor. If they both have the same MFD and breathe similarly, the results could be near identical with regard to the size of the same subject. But if the designs of the two specific lenses are significantly different in how they focus, the results at close distances could also be significantly different.
