Is it a bad idea to solder bullets between the ESC and Motor to make engine changes easy?

It's better to solder and directly to the motor and plug it and plug out there that way the wires can't move and accidentally touch each other I have plugs on almost all my Motors

A) it’s a motor, not an engine
B) what are you trying to accomplish by doing this? You’ll get a better answer to your question if you provide more context.

A) Who called it a engine
B) I want to be able to switch motors easily without soldering. If the ESC has female bullets and all the motors I want to run with it have male bullets, then switching between them is easy peasy.

I think what you're saying is to use a plug rather than bullets so there is no chance of metal touching metal and shorting it out, something like this:

https://www.amazon.com/Connector-Fem...a-828983079081

It's not the best for performance but no harm if you use a good connector. Our club used to use hand out 540 stock motors for the Tamiya class which used bullet connectors. Is there a reason you want to change motors so often?

Not a good reason.

Honestly I figured it would allow me to compete in more classes on the same day.

Back in the day, Novak ESC+motor combos used to come with bullet connectors pre-installed. Speed Passion motors even had bullet connectors instead of solder tabs. Never had an issue with either.

I actually quite liked the SP solution as the motors were relatively cheap, so you could carry a bunch of different winds and swap them out in seconds.

Before I used to bring a soldering station with me I used to solder female bullet connectors to the motor tabs and male bullets to the esc at home so I could swap out and try different motors at the track. No reason not to if you want to.

The Hobbywing Max10 combos use plugs on the speedo and work very well for me in the cars I have them in. The only down side to those combos is that they are sensorless (though I don't suffer with cogging very much on the latest versions it's still noticeable in some scenarios).

It's VERY common for most brands of RTR's on the market to use bullet connectors between the ESC and motor, this is industry standard.

Racers know that adding extra connections will increase resistance and opportunity for points of failure as connectors go bad over time so most racers will opt to direct solder their wiring to the motor.

Back in the 90's most racers would direct solder their battery packs every charge cycle and never used connectors back then, granted the quality of connectors have drastically improved over the years.

There is no right/wrong/best method provided you are using quality components and regularly inspect/replace worn connectors.

If all you’re doing is trying to get more track time, 100% go for it. Heat shrink them really nicely so they don’t touch and you’ll be fine. You can even stagger their positions to prevent them from being able to physically touch as long as you have 3-4 inches of wire length. (Put wire A close to the motor, wire B in the middle of motor & ESC, and wire C close to the ESC)
You’ll be fine. Over time, make sure you keep the male bullets sprung outward for a nice tight fit so they won’t come unplugged during a heat. You can jam a ice pick or something into the center of the bullet to flare the individual pieces out.

Bullets are fairly common for any type of racing using handout motors. You'll add a little resistance to the wiring, but it's likely not a huge concern. Make sure the bullets fit together very tightly. However, if you have a decent soldering station and are competent using it, it really only takes a minute to re-solder a motor.

This would be under the category of 'try it, and judge for yourself if it works for you' ... certainly wouldn't be much of an issue should you decide that it does not, and just switch back to hardwired ...

The best thing to do is get a good soldering iron and quality Kester brand solder. I personally use stuff with 2% silver to make a nice looking connection.

A good iron, good solder and a little flux makes soldering normally MUCH easier. Also I grab a hobby knife or a 1.5mm hex and push down on the wire after you take off the iron so as the wire cools it doesnt pull away and make a bad connection. That and keeping the tip tinned and clean so that solder sticks to it and heat can flow. Sand off any black spots and tin them with the iron at med/low temp just able to melt solder.