I am interested in Yoshi, but at this point it's just "Spring 2019" which with Nintendo usually means June.

I somehow doubt that. Fire Emblem and Yoshi are both listed for Spring, and between the two Fire Emblem is definitely the one that needs the most work (and wasn't even revealed till very recently). Nintendo usually splits new games up by 1-2 months so I'd say April. 

ARMS and Mario Tennis were both "Spring" releases and both landed in late June, so that seems to be their pattern with the Switch. Yoshi is the more high profile of the two and as such seems a more likely candidate for the June slot.

ARMS and Mario Tennis are a year apart .... and I don't really see how that pattern disproves anything considering we have 2 games confirmed for Spring, not just one for Nintendo slot in June. Example being that Mario Kart 8 landed in April. 

Also Fire Emblem Fates sold more than any Yoshi game since Yoshi's Island DS or possibly even earlier (sold 1.84 million months before launching in Europe and only two weeks to six weeks after launching in America) so I wouldn't say Yoshi more high profile. Plus Nintendo's most profitable mobile game is Fire Emblem. 

Yoshi just seems more the classic Nintendo mascot game for June, along the lines of ARMS and Mario Tennis, plus it's being made by Good Feel who took forever to finish Woolly World and are taking forever on this too since it was meant to be out this year. I'll be shocked if it comes out earlier than May. As for Mario Kart, Nintendo is tending to put their Wii U ports in less high profile slots, e.g. NSMBU in January.

I don't know, I guess I just see your take as odd, next year will by far be the most I get out of my Switch personally (also I think an April/May release for Yoshi is extremely likely and the only reason I could see it coming after FE is if they're for some reason worried NSMBUDX and Yoshi won't be good back to back games)

2019 is looking pretty good overall for Switch games I'd buy; I'll be getting Yoshi, Trine 4, Luigi's Mansion 3, Team Sonic Racing, plus potentially games we don't know about yet or which aren't yet confirmed for 2019 like Doom Eternal, Bayonetta 3, Metroid Prime 4. 

And that's the other thing, they already have a 2D mascot platformer coming out in early 2019, they'd likely want to space those out a little. I'd love it if Yoshi arrived in April or sooner, I just don't expect it. 

For those of you who are disappointed in the delay of Team Sonic Racing, it looks like there is a good substitute for it available on the Switch:

https://www.youtube.com/watch?v=xjH58aCpBa8

Second quarter results in about 3 days.

https://realotakugamer.com/bethesda-says-next-wolfenstein-will-be-on-the-switch?fbclid=IwAR3Ao5l7difhH3BEyGo8gwD4DHXgxnwx7k9MO4YTOSPS5wpEpJN4w1_urp4