1. Use PHPStorm. If you contribute regularly to OSS, you can get a free license from them. If not, it is worth its every penny multiple times over. VSCode is nice, but it does not come close in the level of intelligence it has in analysing PHP code and figuring out the slightest mistake or code-smell. It also has great integration oob with the whole ecosystem, from grokking composer files to ofc git, rdbms and ssh support - but it does not force any of that on you, apart possibly for the "work on a project at a time, do not open a file at a time" paradigm.
2. About switching to a framework, as others have pointed out, the answer is "it depends":
- do you need to onboard junior devs? If so, using a well known framework will save them learning time
- is your own code high or low quality? would you throw it away and refactor it in any case, or is it good enough to only require minimal maintenance? if the latter, do not bother rewriting
- being able to rely on someone else's implementation for "tricky" bits such as authn/autz, rdbms abstraction etc... is a benefit in your situation, or is it a hindrance?
- what's the expected support lifetime of your code? can you find a framework which has the same lifetime and is not likely to disappear on a whim? what are its BC promises?
- do you have comprehensive testing, functional+integration, or not at all? if you have no tests, refactoring is a big gamble
- is your code already well separated into SOLID components, or is it an organically grown bowl of spaghetti? If it is, a framework can be introduced piecewise, sliding in one component at a time*
* = this is a bit of a stretch in fact. What you could use to piecewise-replace your own code is generally acknowledged as being a "library", whereas a framework is more of a coherent whole application which calls out your code here and there.
BUT
Symfony comes with both a framework and a set of independent, well tested, maintained and documented components.
A good "middle of the road" choice could be to start using piecewise those components, until all the pieces make sense and click together, and possibly introduce the full fledged framework in a later, final step. Begin with using eg. dotenv for managing the settings, twig to separate the presentation layer, psr/log for logging (and later introduce monolog), adopt an Entity/Repository pattern.
In a recent project I worked on, where the prime directive was "no external code, full auditability is required" I ended up implementing from scratch most of the components mentioned above. Just copying the necessary parts of the Sf components API gave me reassurance about the solidity of the patterns adopted, as well as keeping the door open for possible swap-out-this-component scenarios in the future.