What isn't clear is this: what's in direct sunlight, the ungerminated seeds or the seedlings?

The big warning that I see is you say the ungerminated seeds alternate between moist and dry. This usually clobbers germination rates. You also have the seeds going down to 60F/15C at night, and you might consider doing something to keep them steadily up near 20-23 degrees (for most garden seeds).

The actual mix or material you use for germination is relatively unimportant. That's an anathematic statement for many folks, but your consistency is more important than your materials. .

The ungerminated seeds I leave on a table under our patio which doesn't get much direct sunlight, once they pop up I move them to another table not under anything.

I can probably bring the seeds into the garage which is usually quite stable in temperature, although the temperatures should be rising soon as winter has passed. I will try keep them at a more stable temperature. The low temperatures we've had recently explains the slow growth I guess in the last few months/winter.

Will small cell sizes affect the seed germination rates or growth in the early stages? I know once plants get root-bound it severely limits growth, but what about the first few weeks after germination?

I would imagine your high temperatures can completely inhibit pansy germination. Usually 70F being the maximum, to obtain germination within 10 days. I use a moist coffee filter, placed within a baggy, & transplant as sprouting occurs using a toothpick, into each cell in the tray. Keeping indoors near a window, for indirect exposure to light
Begonia seed, is like dust & likely a very light covering of vermiculite may be helpful, while using a clear dome & cool temps, also. But, no actual experience.
Heartsease= johnny jumps up, or viola tricolor? Do best with autumn sowing.
Not sure if the Phlox you sowed is annual or perennial type. But the annual drummond sort want to grow deep roots, fairly quickly. I can only assume that, as the perennial P. paniculata take about a month, to germinate.
Afternoon shade, may help prevent drying out as quickly.

Just checked the seedlings and omg, everything that was transplanted seems to really be growing up quick! 3-4 days and some seedlings seem to be 5x bigger from around 10mm high to 50-75mm high with massively increased leaf size.

In my good seedling mix I am using a 50:50 mix of Searles premium potting mix + perlite, have added a lil bit of blood and bone, and I am also trialing Bactivate + the seaweed and bioboost (seems to have molasses to feed the bacteria) formula. Bactivate has 5 types of bacteria of the Bacillus family, Bacillus subtilis, Bac. megatherium, Bac. mucilaginosus, Bac. licheniformis, Bac. thuringiensis.

The temperatures have also risen up to the late spring tropical heat, today the rains are sorta starting but not sure if they will last. Usually it's just very dry in the last few months which is extremely strange for where I live as we usually get 4-5m of rain.

Not sure if the boost of growth is due to soil, or if splitting them helped. The original mix I was using was a seedling mix and sort of had a spongy wet appearance but I had also sifted larger particles out which probably didn't help. Trying the seed raising mix by itself seems to not work as good. My 50:50 mix doesn't appear to get as waterlogged, it's also what I used the first time around which had the marigolds, etc grow like crazy. The seedlings in the straight scotts mix in the small blue containers had some rootbound, some weren't, but they were 2-3 seedlings to each container and many seemed to have oodles of room to spread into with the small blue cells.

Phlox drummonii Dwarf is what I was using. Also had 1 pansy popup.

I now have around 80-100 heartseas! I transplanted them into larger 24cell trays and their growth has substantially increased in 3 or so days since repotting.

Dietes Grandiflora popped up after about 2 months, slow to show but they seemed ok so far.

I'll try move the seed table a bit over, there's a shaded spot a meter or so away that shades around 1pm onwards. My next run of seeds I will try stick to the 50:50 mix with bactivate and put fine grade vermiculite on top. Gonna try do bottom watering as well, but finding a way to do that for a fair few trays is a bit annoying.

I agree with the others, keeping the seeds moist and in the right temperature range is a major factor in germination. Some plants are very, very sensitive to temperature. An example, In my grow room Gazania seeds will germinate at 16C, not 14C, not 18C but 16C for best results. YMMV.

I found Phlox to germinate best with my Gazania seeds or 16C.

I have had better luck pre chilling pansy seeds and keeping them about 21C for germination.

Your mix seems like a lot of work and money but if it works for you go for it.

To bottom water effectively I use 1020 Daisy Trays that I place all my cells in. These are then set into standard 1020 trays filled with water. I can water about 20 flats in 20 minutes with this method and 3 1020 trays.

Good luck

SCG

The mix is pretty easy to do, I can makeup a nice big 20-40litre tub of it in about 3 minutes.

The bactivate costs quite a bit but it's around $85 for a 25kg bag, and the application rates are like 15-30grams per m2. I've already done half of my garden beds and have only used about 1kg. They (bactivate) may later on do smaller bags but they are targeting commercial growers first. The seaweed and bioboost+ liquids were about $15 each for 500mL each, I mix at 20mL per 10 litres and use that on about 5-10m2. I put about 30grams of bactivate into about 20litres 50:50 mix I make now to see how it goes.

Perlite I get for AUD$25 for 100litres. 65L of potting mix is about $14

I live in a high rainfall area with lots of fusarium wilt issues, etc so hopefully beneficial microbes in bactivate will help to limit the damage. There are a few videos on youtube of farmers using bactivate and it seems to have amazing differences in growth but I guess time will tell.

Yeahp, too much work for me LOL! I used to do all that stuff but now use what I can easily obtain.

There is a good article on the container gardening forum about how much perlite or the like you would have to add to really make a difference and YMMV.

I am interested in the Bactivate, possibly just because I have a curious mind, but also wonder if it is like the mycorrhizae.

I did try 'Serenade' a good bacteria spray this year but have not used it long enough to really comment on its effectiveness.

Keep us informed on how you make out.