To save your Deezer collection to a JSON file, open your favorite tracks (like https://www.deezer.com/en/profile/1725967/loved) and run this script in the console.

save-deezer-collection.js

(async () => {
	const keys = [];
	const songs = [];

	const index = () => {
		const rows = $('.datagrid-row');

		rows.each((i, el) => {
			const row = $(el);
			const key = row.attr('data-key');

			if (key && !keys.includes(key)) {
				keys.push(key);
				songs.push({
					key,
					title: row.find('.cell-title').text(),
					artist: row.find('.cell-artist').text(),
					album: row.find('.cell-album').text(),
					duration: row.find('.cell-duration').text(),
					added: row.find('.cell-date').text()
				});
			}
		});
	};

	const wait = (ms) =>
		new Promise((resolve) => {
			setTimeout(resolve, ms)
		});

	const walk = async (cb, step, delay) => {
		window.scrollTo(0, step);
		await wait(delay);
		cb();

		let scrollY;

		do {
			scrollY = window.scrollY
			window.scrollBy(0, step);
			await wait(delay);
			cb();
		} while (scrollY !== window.scrollY);
	};

	const save = () => {
		const json = JSON.stringify(songs, null, '	');
		const blob = new Blob([json], {
			type : 'application/json'
		});

		const url = URL.createObjectURL(blob);
		const link = document.createElement('a');

		link.href = url;
		link.download = 'deezer-collection.json';
		link.style.display = 'none';
		document.body.appendChild(link);
		link.click();

		setTimeout(() => {
			document.body.removeChild(link);
			URL.revokeObjectURL(url);
		}, 0);
	};

	await walk(index, 500, 200);
	save();
})();

Ok this was stupid!
There's a public API: https://api.deezer.com/user/1725967/tracks