Pranams suryakasyapa sir. Everyone are halfbaked only. Even, the potter GHOR KUMBA said Namdev as Halfbaked. Am then quarter baked. Anyhow I will share my thoughts regarding this issue.
Sandhya is actually twilight period. As you said its joining (Sandhi- words joining in sanskrit). Here two periods are joining. (Night-day, Day-noon, noon-night).
If we wish to do sandhi, it is here (Kaanamal Konamal kandu kodu). It means provide arghya to surya before he rise (Arunodaya punya kala), when he stands straight (Mid noon), and before he vanishes (Evenings). Sandhi is the doing , actually a wordchange (Maruo- in tamil) of sandhya vandanam. We have to think soorya and, savitri, gayathri and saraswathi who are the three wives of Brahma. Also, it is customary to think these devis as brahmi, vaishnavi and maheswari during sandhya period.
Surya kasyapa sir, Iam not saying that rishis developed sandhi for chanting namas. I said this statement, "in relation to kesava, narayana, madava govinda... alone". for that alone, they too will say siva siva siva. Otherwise all the mantras are very powerful. Apart from chanting divine namas there are so many things.
1. Doing prayaschitha for yesterday sin (manasa, vacha, hasthapyam, pathpyam utharena chichnam) all these kinds of sins are washed off by doing sandhi.
2. We get cleanliness and holiness (Abohishta mayo puva... Thathikravinno agarishma...)
3. Doing our rights to pacify devathas and saving soorya from "Mandehal- a kind of demons" (Our arghya theertha will turn on vajrayutha and kill the demons who are near the chariot of soorya) by providing arghya
4. Then Sandhi (Anganysya, karanysa, dhyna, japa, raksha)- This are all small rehearsals for greater penance.
5. Then upastana and dikvandana. Here, our sandhi teaches us to pray to all the directions saying the glory and importance of each direction. We prostrate to sages who conquer lust and anger. (indierctly it urges us too live like that. So only no names were mentioned there). Yama, narmada, hasthika, jaratkaru, sarpadevatha, chitragupta, Viswaroopa are all worshipped there. Finally, we prostrate in front of soorya.
6. Then a papa prayaschitam to narayana (kayena vacha...)
There are these many things sir. So, Iam saying that simple nama chanting alone. Whenever we do nyasa on body parts, we says kesava...dhamodhara, instead they will say siva siva... Thats all. So many things said there are very common. Moreover sandhi is common for all. It is our duty, not to pacify anyone. NITHYAKARMA. You would not get anything by simply doing duties. But, you will become responsible, active, sharp, bright and so on...
Hats off to SANDHI
"Sandhi seiyada moodargal mannil mandhi (a kind of monkey) kku samamame"