I was thinking about software to create, sign or decode transactions without the Node. To create a transaction is an algorithm where we have to input the TXID, VOUT, and the Amount. And to sign it we need the private key.
A long time ago in order to understand bitcoin better I started a project called
BitcoinTransactionTool. It can decode, modify and create transactions by receiving UTXOs from block explorer APIs. Feel free to look at the code but know that it is incomplete, buggy and abandoned but it works well for basic things such as decoding basic transactions and creating legacy transactions.
I have long moved on to a newer and more complete project which I call Denovo and will eventually add this option to it. Unlike the old project, basic things such as transaction/script decoding and evaluation are complete and well tested here. It just lacks the user friendly way of creating or modifying transactions and it lacks the option to get UTXOs (Full node or SPV clients are incomplete and won't add centralized options such as block explorers).
I'm sure we need a Bitcoin Node to push the transaction
You don't really need to run a full node to broadcast a transaction. All you need is a peer IP address (could be gained using DNS dig), an open socket to that IP and ability to construct version/verack messages and sending them.
Denovo has this feature in a
simple UI which uses what I call
MinimalClient.