A fully synced Bitcoin node is the only way to know without trust that the coins you think you own are really yours. Otherwise, someone can trick you into thinking you have money you don't. You can try to spend the counterfeit coins, but the network will reject your transaction.
If disk space is your primary concern you can run your node with pruning active, which deletes old blocks from disk but maintains a complete UTXO set, which is all you need for full validation.
If bandwidth or CPU is your primary concern, you can investigate the SPV and Neutrino models, which compromise privacy and security in varying degrees for convenience.
Finally, and this is not recommended, but you can trust someone to run a full node for you and provide you with the minimal amount of data you need to create a transaction which you can then sign and broadcast locally. This can be done (for example) by requesting transaction data from Blockstream's esplora API and you can see an example of such a construction in my (novelty-only-do-not-use-for-real) brainwallet sweeper application: https://github.com/pinheadmz/brainsweeper