Hmm, I've listened to Mainsail Part 1 a few more times, and changed my opinion on a few things. Do note that the only reason I'm digging into this song as much as I am is because I like it a lot and see a lot of potential in it, but potential I feel hasn't been completely tapped yet.
Scrap my previous comments on the intro. I think I mostly want it to make that initial changeup closer to the 18 second mark rather than the 21 second mark. The slow, teasing lead-in does work, but it's just a touch long to my tastes.
For level adjustments, the electronica accent first used at the 24 second mark could go up a little, while the synth, bass drum and that wood percussion thing can all go up more. The high hat is basically at the level you want it, just need to bring every else up in line with it comparatively. Still might be hot presence-wise, and I'm not sure if the levels on the bass guitar would need adjustments or not. Which is to say, the song is mixed soft compared to industry standard these days, and Part 2 is even softer.
The remaining weaknesses are more structural, so this is where I'm likely to shoot myself in the foot when I get on this soap box of mine. You've got half a foot in progressive style tracks and one and a half in composing music with Mainsail Part 1. By progressive style tracks, I mean layering in instruments as time goes on without much changing what said instruments are doing. It's a cheat, basically, where it's typically slowly warming up for the first 80% of the track and then has something worth turning into a song for the last 20% of the track - at which point, said track just ends.
What I would like to see is you step fully into composing with Mainsail Part 1. To tell you what I mean by that, I dug up my favorite song in this genre (Solar Sailer remix by Pretty Lights) in the off chance I might manage to say/show something intelligent with a comparison. And by 'theme', I basically mean the melody line plus associated accents below.
Composing music differs starkly from making progressive tracks in that composing involves developing and exploring the theme (melody line) and arranging those developments and explorations into a cohesive whole. For Solar Sailer, that's both the structural blocks arranged as shown along with the multiple Theme Variations, and for Solar Sailer, it also involves hinting at the next Theme Variation ahead of time and other things that I'm sure mostly fly over my head at this point.
Mainsail Part 1 isn't quite there yet, so first some more specific critiques.
Intro - I think you made a slight hash of your intro with the progression of the elements. It kinda feels like the bass guitar comes in too early and steals some of the thunder from the little flourishes that get added to the melody at 0:30. So I'd say you could play around here some more to get a bit more from the intro, and along the way, the intro as a whole could be lengthened a touch too. Do quite like the bass guitar coming in here though.
Dropping in that snare at 0:58 is kinda like a weird, almost transition between the first theme variation and the interlude... I'd say this is one of those progressive track missteps subbing in for a proper transition.
Interlude v2 + Theme - This is almost a Theme Variation 2, but falls a little short. On one paw, it's the interlude with a different snare and the theme dropped into the background, and on the other, it's basically using the raw version of your melody line which works better with the breakdown rather than as a variation of the theme. It's not bad, and after some thought, I think it could work well if it was separated from the first interlude and used as a distinct second one, probably transitioning into the breakdown as now.
The last 10 seconds seems like you put some higher energy instruments back in just to have it all run out of steam. It's not a particularly satisfying end. Simply leaving the breakdown to ghost out piece by piece woulda worked better, methinks.
But I think the biggest takeaway here is that Mainsail Part 1 is two Theme Variations and a proper Outro away from having all the basic pieces of Solar Sailor, which would extend it to that 3ish minute song mark. To get closer to 4 minutes, you could extend all of those pieces (sans the interludes) without them overstaying their welcome and possibly repeat a section like Solar Sailer does. You already have a solid core for a great song, so again, would be pleased to see it fleshed out fully.
I will be seeing about those EPs later, I think.