Welcome Tob! I´ve played some of your levels, here´s what I think.

The real Doomship: Honestly I´m no fan of overly hard levels. I spent about 15 lives and got as far as the first ice block onboard the ship before giving up. I got no enjoyment from this level, so I tried some of the easier ones instead.

Downpath Hijinx
This is a fun New SMB-style level, actually feels a lot like it belongs early in one of those games. You´ve got a good blend of enemies, having just one big Chain-chomp makes it more memorable. I like that you´ve hidden an extra life near the top, makes exploration feel more rewarding.

Treasure Tracker 1 is a good easy puzzle, I like the Treasure Tracker-style of levels. Check out Joshua’s post in this thread for some more nice Tracker-levels! My one concern about your level is that you could reach a dead end if you don´t take the coins once the P-switch is activated. You only get one try and must reset the level if you didn´t think of it. Most players will however.

Goomba Mining Facility
Nice level with a reasonable challenge. The solution with the big Koopa shell is fun, and the frequent use of Goombas gives the level personality.

Yoshi’s Island Maker
Yeah, I sure wish there were a Yoshi’s island theme for SMM… Pretty straight forward level, the goofy sounds when you collect coins fit the Yoshi theme well. Also, the big Buzzy beetle reminds me a bit of that crush-rock in Yoshi’s Island.

Pipeline Panic is one of my favourites, good balance of difficulty. I had fun dodging piranha flowers, and the jump from the vines were thrilling. It can be a little trial and error since jumping down from a pipe can have a surprise piranha flower can bite you in the back.

Deep Sea Dive
This is an easy level which is fine, it could be more condensed though. There are some stretches where very little happens. I found one difficult passage, the placing of the big fish which is almost impossible to avoid as big Mario. The gate of munchers near the start is a good obstacle, but you could render the level unbeatable if you fail to catch the mushroom. The bouncers on tracks is a creative obstacle when I get to try running away during the P-switch. The first set of bouncers feels kind of pointless though, since it´s basically empty. Perhaps there could be coins to collect or a power up block?