You could run an open grate, (ditch), around the perimeter of the porch and at the bottom of the ditch run a french drain set up that leads out and away from the area. It would mean saw cutting the concrete around the perimeter, digging out and installing the french drain, (perforated drain pipe), laying landscape cloth and gravel over the french drain. I would go about 12" in width on the ditch and you can use gravel or decorative stone such as small river rocks to top over the drain. I guess another alternative would be to jack hammer out the existing sidewalk, grade away from the structure, and repour or install pavers.

g'day cestrum, generally most downpipes are covering 50 to 100 sq/mtrs of roof so i would have expected you to have at least 2.5k litres of water in the tank by now. neighbour over the road ahs one of those 5k/lt tanks he has app' 100 sq/mts of roof feeding it so i would expect his tank to be full by now, he's been sitting on an empty tank for the past couple of months though. not sure which water diverter you are using but sounds like it could be set up better, we are using 2 of those floating ball cock type systems that are just a bit of downpipe in our case 1 meter long that holds app' 7 litres of water from the first flsuh of rain, we have so we drain them manually. after the first capture and with continual rain there is no need for continual first flushing to occur, we have 2 for our 25k/lt tank to cover the app' 100 sq/mt roof we get water from and this is heaps of first flush capacity. and with rain like this i won't empty those flushers until the tank is full. up in rural we never had first flush units as we needed every drop that fell. the water that comes out of ours is always crystal clear very little solid flotsum in it and gets used to wash clothes or water the garden. if you tap on the side of the tank with your knuckle you can here the difference in sound tells you how much water you have, but with your tank probably just as easy to have a look through the top. got pic's of our set up on our site. len Here is a link that might be useful: len's garden page

Once you landscape, you should solve most of your water problem. Adding exterior water proofing and drain tile around the entire foundation to exit on the down hill side will solve the rest of the intrusion into the crawl space.There really are no shortcuts in dealing with active water intrusion into a crawl/basement space. Preventing the water from entering is really the only sure way to maintain a dry space. As for the radon abatement, there are lots of solutions. Passive abatement is the least expensive long term. That means installing a grid of drain tiles(holes downward) and a stand pipe(extending to a height above the roof) under gravel. Then the plastic you mentioned and pour the concrete floor. The passive system is done without the concrete, with the plastic sheeting glued to the foundation walls and any pipes. Pouring a floor over it is fine., but the sealing is still very important, as concrete alone will not effectively seal against the gas. Which is forced from the earth naturally by pressure in the earth. Even though radon is heavier than air, the passive system does not allow the gas anywhere to go save into the piping and eventually up the stand pipe and out into the outside.

There are a number of ways to create a ceiling under a deck to create a dry space. The simplest are the ones that come as a system that you attach to the underside of your framing, like this one here: https://www.zipupceilings.com/zip-up-underdeck/ There's another system we've used but I can't remember it off the top of my head. You can also use 4x8 sheets of AZEK pvc trim lumber, using shims to pitch the water away from the house, mounted under the deck. There's also a system called rain escape (I think) that goes down on top of your joists BEFORE you install the deck boards. In all these options, you're pitching the ceiling away from the house (typically). There's a gutter attached to the inside face of the beam, and then a downspout attached to one of the posts. As far as best price, focus more on where you'll get the best service if there are any manufacturer issues. Local lumberyards or building supply yards are the only places I would ever buy composite decking.