Deck drains = to hard flex tube = out to Gemlux Flapper scupper drains. I also tied in my transom live well drain out. Just some thoughts for you.

Any questions?

What size lines? Dia?

As I recall, they were 2" Bilge hose to the 1 1/2" glassed transom deck drains. I used a glass exhaust reducer.
The same size hose as the Jabsco Macerator Pump.
The Gemlux Flapper thru Hull Suppers were 2" Barbed. If you heat the hose a bit it goes on & off easier, then hose clamp.

I was thinking about 1 1/2" but I know the 2" drains that were on the 27 drained out fast and I never had anything stop them up either. I was looking at some glass tube on ebay and may try to glass the whole span with one long tube so that I can get away from the hose and connections. Seems like it is only 18-20" to get out the back from the inside liner. I may test this on the 25 first and see how solid tubes work out as that hull will defiantly get drains as it will see some offshore use.

https://mgs4u.com/product-category/fiberglass/tube-rod/

I ordered some 2" ID fiberglass pipe from this website and glassed them in from my cockpit gutters out the transom. very strong, fast shipping, and cheap. Not even my camphor trees can clog them up.

Attached are a couple pics before glassing them in.

Yes, I think your OK with that thought of a solid drain tube.
I did try that route first, but got nervous about the stress on the tube considering I was altering its circumference strength with adding the rod holder drains to it. Also the flex with the bracket & the motor weight on the transome. The angle also was not right from the gutter config. I wanted the Gemlux Flapper Scuppers which would not work with the solid tube. I hate water in the bilge. You can see what I'm talking about in the picture.

The boat flexing would be my major concern. I would think if I glassed them in with epoxy it would have enough flex not to crack out hopefully. I do need to check the line of sight and angle of the transom to make sure it is doable though. I don't have tabs on the 20 so it shouldn't have much blocking it.

I did almost the exact same thing on my 79 Sceptre.
Worked great.
Been three years and no signs of flexing or cracking.
I used West System G-Flex epoxy which is a little more flexible than straight up West System and I used 1/8 inch wall 1.5" prefab structural fiberglass tubing from mcmaster carr.

https://www.mcmaster.com/8535k72

https://www.mcmaster.com/mvA/Content...=ImageNotFound

5 foot chunk, 26 bucks. Hard as nails.

1.5" ID works fine. 2" ID I guess would be better, but for my use and with my slightly raised deck, the 1.5 does the job.

Covered them with these:
https://cdn.shopify.com/s/files/1/18...g?v=1547699502
T-H Marine rectangular scupper.