@TonyVan , as the others have written, what you may see through the optic - i.e. the "interference" you wrote of and are concerned about - is dependent in large part on the particular optic you use.
With low-powered variable optics (LPVOs - aka adjustable magnification "scopes") as cited and pictured by
@Talyn and
@HansGruber, respectively, the way the optics of the optic
works effectively "ghosts" and then completely eliminates the obstruction that your FSB poses, even though when you look at the gun from the side, you see that the FSB is in the line of the optic (
@HansGruber 's picture demonstrates this very nicely).
With a non-magnified "traditional" red-dot (to make things easy, I will include both the "reflex" and "holographic" sights in this group) at either absolute or lower-1/3 co-witness, when you drop your line-of-sight down to the plane of your iron sights and look through your RDS "red-dot sight", your FSB will remain fully visible just as though you did not have the red-dot in place at all. For shooters with astigmatism, this setup of (eye)-(rear sight)-(RDS)-(front sight) is often helpful to effect more precise shots as the rear sight aperture helps (via optical principles) to suppress the "fuzzing/distortion" of the dot that shooters with astigmatism often see, when simply directly visualizing the aiming dot. At-distance, if the dot chosen is refined enough -i.e. a "one MOA dot"- and the shooter was diligent (and capable, in terms of how well he/she sees) enough to have very carefully sighted-in and co-witnessed that dot, this kind of setup could potentially even help further refine the iron-sight picture, as the front post width typically brings a level of uncertainty into the mix.
With a prismatic red-dot, instead of a "projected" reticle such as those seen on "traditional" RDS, the reticle is etched into the glass of the optic in the same manner that the reticle is etched into the glass of "scopes." This means these markings will remain sharp even for those shooters with astigmatism. However, there are compromises. While the reticle can be clearly seen even by those with astigmatism, it takes more battery power to illuminate these markings (leading to shorter battery life) and typically only the better makes/models offer true "daylight bright" illumination (but it can be argued that this isn't totally necessary, as the etched markings will remain visible even in the brightest sunlight). And unlike the relative freedom traditional RDS bring to the sight picture, the prismatic sight has eye box and eye-relief concerns (while this makes the sight less forgiving to use when shooting from compromised positions, the way these factors force head/eye alignment behind the optic also makes the system less prone to parallax, which, despite the "zero parallax" claims of many "traditional" RDS, still remains an issue. Finally, because of the prismatic optics, even though the FSB may align with the optical axis of the sight, it will not resolve with sufficient clarity to "co-witness." Further, depending on your definition of "interference,"
@TonyVan , the residual "shadow" that the FBS casts into the sight package may or may not be acceptable to you.
Overall,
@TonyVan and
@MagaTex , if you can accept the expense of the LPVO, my recommendation is to go this route. The reason is not only because it makes the FSB a non-issue as you increase magnification for longer-distance shots, it also makes every bit of "seeing the target" just that much easier for those of us with vision issues or are simply getting older. While the LPVO itself doesn't necessarily have to break the bank, the addition of a good mount will typically add somewhere in the neighborhood of at least $100 to the budget (unless you pursue traditional two-piece scope mounts).
Weight/bulk wise, the modern LPVO and mount base will be no worse off versus the addition of a magnifier behind the traditional RDS (magnifiers are not optically compatible with prismatic sights), and in many cases, the addition of a magnifier to a traditional RDS pushes the cost to-parity with comparable-quality LPVOs, and this is even more the case when the magnifier is paired with a "flip-to-side" base that swings the magnifier out of the way when the shooter desires 1x view through the RDS.
If a shooter has astigmatism, another good choice is the prismatic, however, one needs to realize that in-use, even the "1x
prismatic won't be the same as a traditional RDS. Similarly, although the weight and bulk compromise will favor the prismatic versus the LPVO, the prismatic is a fixed magnification system.
In terms of viewing a magnified front sight through a magnified prismatic or even a traditional RDS's magnifier, remember that there will be a noticeable visual degradation of the "crispness" of the front sight's visual appearance. Additionally, even if not visually distorted, the front blade will "magnify" in proportion with target/sight picture, and thus may not offer much visual advantage.
Finally -and I often find myself shying away from saying this, because we all work hard for our money, and most of us have limited budgets- buy the best that you can.
Nowhere else will it be as immediately obvious to you how every penny you've spent translates to quality. Edge distortion, clarity, color shift, fisheye, etc. - particularly for those of us who are specifically looking at using optics in order to offset our vision shortcomings due to disease and/or age, if we're already looking to overcome issues to begin with, it makes sense to try to minimize any further shortcomings as much as possible.
Luckily, a lot of good choices are available these days, and shooters should also look at the secondary market for used items, as many previous-generation optics offer exceptional value.