Whether or not just using a reducing bushing is code-compliant depends on the type of electrical box and use involved. If it is just a "metallic outlet box", then a reducing bushing also listed for bonding use (as apparently most are) is OK, even if all the concentric rings in the knockout have not been removed. If the box is some other enclosure than a metallic outlet box, you must remove all the concentric rings in the knockout in order to be able to use a reducing bushing there.
However, if service conductors are involved, you cannot use a reducing bushing but must instead use a specially-designed and listed bonding bushing. In these cases, according to my reading, you must use a special bonding bushing every place you have not removed all the concentric knockouts, as the connecting metal between rings has apparently not been tested and listed to handle a high-current fault situation as with service conductors.
From the size of the wires and conduit you mention, it sounds like you may have a service conductor situation (and certainly not an outlet box situation). Thus, not only for the damaged knockout you mention, but for all the other knockouts in use that have not had all their inserts removed, you may have to use a special bonding bushing.
The above info is based mostly on a QA in IAEI Magazine:
Question
If I have an installation that is 120 V or 208V and use a reducing
washer in a concentric knock out in a panelboard or safety switch but
do not remove all the concentric rings, do I need to install a bonding
bushing?
Answer
Yes, you would need to use a bonding bushing that is certified
(Listed) as Grounding and Bonding Equipment (KDER). The Guide
Information for KDER can be located on page 262 in the 2014 UL White
Book and can also be found on UL Product Spec at
www.ul.com/productspec by entering KDER in the UL Product Category
Code search field. The Guide Information states that grounding
bushings for use with conduit fittings, tubing (EMT) fittings,
threaded rigid metal and intermediate metal conduit, or unthreaded
rigid metal and intermediate metal conduit have a provision for the
connection of a bonding or grounding wire or have means for mounting a
wire connector available from the manufacturer.
This is a NEC 110.3(B) issue. Reducing washers are certified (Listed)
under Outlet Bushings and Fittings (QCRV), located on page 391 and 392
in the 2014 UL White Book and can also be found on UL Product Spec at
www.ul.com/productspec and enter QCRV at the UL Product Category Code
search field. The Guide Information for QCRV under the “Grounding”
heading, states, “Metal reducing washers are considered suitable for
grounding for use in circuits over and under 250 V and where installed
in accordance with ANSI/NFPA 70, National Electrical Code. Reducing
washers are intended for use with metal enclosures having a minimum
thickness of 0.053 in. for non-service conductors only. Reducing
washers may be installed in enclosures provided with concentric or
eccentric knockouts, only after all of the concentric and eccentric
rings have been removed. However, those enclosures containing
concentric and eccentric knockouts that have been certified (Listed)
for bonding purposes may be used with reducing washers without all
knockouts being removed.” If the concentric or eccentric rings were
not removed, you would need the bonding bushing to bond around those
rings.
The only enclosures containing concentric and eccentric knockouts that
have been certified (Listed) for bonding purposes are certified
(Listed) under the product category Metallic Outlet Boxes (QCIT),
located on page 389 in the 2014 UL White Book and also on UL Product
Spec at www.ul.com/productspec and enter QCIT at the UL Product
Category Code search field. The Guide Information for QCIT states
under the heading CONCENTRIC OR ECCENTRIC KNOCKOUTS: “All boxes with
concentric or eccentric knockouts have been investigated for bonding
and are suitable for bonding without any additional bonding means
around concentric (or eccentric) knockouts where used in circuits
above or below 250 V, and may be marked as such.”
Edit: How to install a bonding/grounding bushing? First, the bushings I've seen are UL listed for use on a threaded end, so threaded rigid conduit or threaded IMC, or the threads of a male EMT end fitting. It goes on over any locknut securing the conduit end, and must engage at least 2 threads to have a good bond to the conduit. The lug is then used to bond the bushing via jumper or direct lay-in to appropriate ground.
A very helpful video showing this on YouTube is "Solar Snack - Grounding Bushing Wiring"
Still photo from linked video showing a grounding bushing installed. Note the lay-in lug at 6 o-clock and also partially obscured set-screw at 9 o-clock for making secure physical/electrical connection:
Edit2: Remember that the lay-in wire or jumper connected to the bonding bushing must be sized to handle the maximum expected fault current, so don't just use a 12g grounding pigtail. Here's a diagram with jumpers sized according to the service and their function:
(source: https://www.ktrassociates.com/2020/05/13/whats-the-difference-between-the-main-and-system-bonding-jumper/ )
