How To Transfer Coins From Bitcoin Core to Electrum to avoid Sync Process?
To relieve your space problem, You could either run it on an external drive, or run Bitcoin Core under prune mode. Depending on your OS, you'd need to find a file called bitcoin.conf, modify that, and restart your Bitcoin Core Wallet process. The file would look something like this

The line for prune mode reads

# Enable pruning to reduce storage requirements by deleting old blocks. 
# This mode is incompatible with -txindex and -rescan.
# 0 = default (no pruning).
# 1 = allows manual pruning via RPC.
# &gt;=550 = target to stay under in MiB. 
prune=550 &lt;- change this number if needed


Electrum is a light-weight wallet that relied on trusted nodes for transaction information. This page explains the steps needed to migrate from Bitcoin core to Electrum. According to that, you'd need to dump your private key from Bitcoin core and sweep that into Electrum.