How would I go about making a semi-solid platform?

I haven't tested this idea, but I think you might be able to pull it off. Each of these semi-solids should have CanCollide set to false. Tag all of your semi-solids with CollectionService and on the client, loop through them and listen for Touched events. Inside the client touched event, check for collisions with that client's character's feet. Wait a preset time (this will require a little tweaking) and locally set CanCollide to true. You'll also want to check for the player ending their contact with the part and disable CanCollide accordingly. Since the client owns the character's position, they will appear to be standing on top of the semi-solid to all the other clients as well, even though their local version of the part has CanCollide set to false. If you need further clarification, I can write out this idea and test the code. See below

Tag Editor: https://www.roblox.com/library/948084095/Tag-Editor

Mark your semisolids with a tag called "Semisolid".

Implementation located in StarterPlayerScripts (yes, it's local):

local cs = game:GetService("CollectionService")
for _, v in pairs(cs:GetTagged("Semisolid")) do
	v.Touched:Connect(function (p)
		if p.Name:find("Foot") and p.Parent == script.Parent.Parent.Character then
			wait(0.1)
			v.CanCollide = true
			coroutine.wrap(function ()
				while wait(1) do
					local stillTouching = false
					for _, part in pairs(v:GetTouchingParts()) do
						if part.Parent == p.Parent then
							stillTouching = true
							break
						end
					end
					if not stillTouching then
						v.CanCollide = false
						break
					end
				end
			end)()
		end
	end)
end