As usual this is a large debate. Some like to take the simple approach and use a baffle or two. These people use very little biological and mechanical media, IMO. Their concern is with making the water volume larger. Personally I don't see the point of this. Yes your adding water volume, thus diluting ammonia, nitrites, nitrates, etc., but that's the issue; all you are doing is diluting, not removing or converting. The way I see it is, you have went through the work of setting up an overflow, getting an extra tank, and plumbing everything. Why not use this space to the best of your abilities?
First just a little background, I have had a sump on my 55g dirted low tech tank for the entire 2 years it has been running. Originally I had a 20g long as the sump and I had only two baffles, one for the intake and mechanical chamber, and another for some pot scrubs (submerged). I had this setup for about a year, and it didn't seem to clean the water all that great. So I tried something else, thus v2 was born. V2 was 2 10g tanks, why I didn't reuse the 20g long is beyond me (mistake). This is what the first 10 looked like
Chamber 1 had mechanical filtration (a course sponge, a fine sponge, and quilt batting). Chamber 2 had pot scrubs, and 3 had some Seachem Matrix. With the rest of the matrix in chamber 4 with the return pump. The second 10g was added later for added water volume and to reduce the time between needing to add water do to evaporation. The second 10g was connected to the first with a water bridge, I also use the second tank as a fry tank. This setup lasted up until last week when I rebuilt the 20g long because the water bridge between the 10s kept loosing siphon.
Sump v3 reused the previous 20g long but added 2 baffles and 2 chambers. The reason for this is I wanted to get as much contact between my biological media and the water as possible. I also have cory cats in the display tank, and when they lay eggs (every 2 weeks), the fry were getting sucked down the overflow and deposited on my mechanical media which was only submerged under the water about 2 inches. This left them very little room to get out of the intense flow and not many survived, I wouldn't find them till my bi-weekly mechanical media cleaning. Don't get me wrong they love the flow but they were getting beaten by the turbulance. So I wanted my first chamber to be just the drain and the second chamber to house my mechanical media. This is the results of my designing
So as I said the first chamber (left to right) is empty, then the second has my mechanical media on a raised piece of lighting diffuser. The third chamber houses my submerged pot scrubs and the 4th houses 1 gallon of matrix. The final chamber houses my heater, return pump and extra biological media for setting up new tanks. The sump is the sealed with acrylic lids, the pump and drain go through bulkheads. Sealing the sump reduces co2 off gassing, by creating an atmosphere, co2 is heavier than o2 so if disturbances are kept at a minimum co2 will fall and o2 will rise. The seal doesn't need to be perfect, the drain is going to suck down some air so you don't want hinder that.
This design is very easy to clean, I drain the sump so that the water level is below the second baffle, this prevents the dirty water, created by removing the mechanical media, from getting into the rest of the sump, thus keeping it clean. I then remove the mechanical media, clean it and replace the worn out batting (usually lasts about 3 months), if needed. With the mechanical media out I can drain dirty water, from the first and second chamber, after checking for fry, and refill the entire sump while replacing the mechanical media. Then it's ready to go for another 2 weeks, like I said this is a low tech tank so that's all the water I change during water changes, except for a 50% change every 6 months. And this tank runs 0 ppm ammonia and nitrites, and nitrates stay under 5ppm consistently, even after a month without cleaning.
Now a little advice, Seachem Matrix is simply pumice stone, you can buy it in 15 lb bags from here
for $25 shipped, and 15lbs would probably filter 3 of your tanks no problem, never hurts to have extra though. FYI K1 Kaldnes is impractical in a planted tank because the use of air stones reduces co2 levels, and its overpriced. Lighting diffuser is available at lowes, home depot, etc. for like $10, also called egg crate. Your current design with the wet dry setup can work, its what Tom Barr uses, but the entire sump needs to be meticulously sealed and this makes it hard to clean. Your algae issue is because your using a wet/dry filter, i.e. any co2 added is then gassed off.