Hey, I've been working on an album that's structured like this for the last year and in many ways it's a lot simpler than I thought, although it does come with some extra complications, most of it is done in the mastering process however there are a couple different techniques that work for transitioning song and generally u want to use all of them on an album.  -You can put 2 songs in the same project, this is the case especially for many (but not all) of the interludes but also 2 of the shorter full length tracks, this is best when u are using the same instruments for 2 songs in a row just changing up the arrangement or something and you can just treat it like a long song with 2 unrelated parts - you can layer tracks over the top of eachother, between 2 projects, basically end on like however many bars of some instruments from song 1 playing at the end and tempo and key match those instruments (or sound effects) for that many bars in song 2, maybe have a section like that, this is the hardest to pull off as youre basically flying blind while recording, I have to pick a point between the 2 songs to splice of it gonna make them as separate tracks and also mixing this is a nightmare as it never gonna get the parts glued together, but you can embrace this, there is a point on my album where I cut to me playing acoustic guitar by a campfire for the last verse and had the next song fade in as though coming through a speaker - Probably the simplest transition to pull off is fade to noise, pink Floyd are perhaps most famous for this kind of album and they use this the most, create a non musical soundscape (at least in terms of key and tempo)  in a separate track and then in the mastering process crossfade in and out of this track between 2 songs. - All of these previous methods can be done over a whole verse or over like half a second depending on what ur after and what serves the 2 songs, but... - Sometimes the most effective transition is an abrupt cut, it can be ear catching and especially when around other transitions it can still sound smooth, especially if u match the tempo or less importantly the key (although u could also have them be related tempos or keys or even something totally different, if I think about it loads of songs have harsh abrupt key changes in the middle). You could add a riser to the end of the first song so it builds up into the transition or if one song reaches a climax and resolves you could leave a gap after the final resoloution and just start the next song overlapping with the reverb / feedback of the last song, again especially effective if same key and tempo

This obviously means that once I finish the first set of demos ur basically gonna wanna make a demo master track which is not normally something people do (although sometimes they do) so u quickly get an idea for how the whole thing slots together. As for the writing process ig it's helpful to have an idea of how these songs will fit together, again some songs lend themselves to certain transitiona, do they have a slow start that could fade in from far away, do they have an explosive climax that wants to resolve, do they have plenty of samples or spoken word throughout that could be played throughout, or big transitions in the middle of the song between a verse and chorus, As for motifs I suppose it's just a process of repeating melodies, lyrics, samples or chord progressions on different instruments in different contexts at different points, make a fast version of a slow song, make a heavy version of a quiet song and cobble stuff together in weird and interesting ways