Say I want to mark a 30 blocks radius for spawn proofing, the I would need some kind of Indicator to show the 30 blocks radius,any suggestions?

Minihud, as mentioned by Ginkgo is the most common approach, but if you want "vanilla", you can use "Pixel Circle Generator" on-line tool, and follow the generated edge.

Enter "width: 61" (it uses diameters, not radii) then follow - 30 extra blocks away from the central block (easiest to do: prepare a stack of 30 blocks in your hotbar and build a line until you run out) - then follow the picture - 5 blocks out to a side from the end of your radial line, 1 in and 4 to the side, 1 in, 3 to the side, 1 in, 2 side, 1 in, 1, 1 in, 2, and so on. Working in windowed mode (F11) or with the pattern on one monitor, game on another is the easiest way to do it.

Or much easier, (for purposes of spawn-proofing it doesn't hurt to spawn-proof too much) - mark out edges of a 61x61 square centered on the central block (again same trick, 4 stacks of 30 items), then spawn proof it taking good care to adhere to the borders near middles of the sides, while being completely sloppy around the corners.

Note:

Shulkercraft is a content mill channel which is mainly copying content off other creators with very little understanding of what they they do, and whatever they produce themselves has usually quite lackluster performance. This farm is no exception.

In particular, this location of the afk platform (directly above the center of the farm) will severely cut into the spawn rates. Lack of pack spawning extensions also heavily limits the rates, plus the flooding of the platforms as soon as even one witch spawns is another severe factor making it one of the worst designs out there. If you want a relatively easy design, pick Xisuma's, LogicalGeekBoy's or maybe Ray's. If you're more ambitious, Ilmango has excellent rates. Witch farms have poor efficiency as they are, and so using Shulkercraft design which is a good 30-50% worse than the next competitors and far behind the leaders - is hardly worth the effort.

You can use a mod such as MiniHud, which can overlay spheres/cylinders/circles with a specified radius into your Minecraft world.