The question is why a company would wish to hire someone who is only available during a short window in late afternoon for sales? That needs a stronger answer than "I'm a developer working on an unrelated product who would like to gain sales experience but only as a side hustle", because sales engineers and product managers who love talking to clients are willing to make themselves available when the clients want the meeting, not when they've finished their real job.
The more obvious way for technical folks to break into SaaS sales is to be interested in contributing to the sales effort of the company that also employs them to write the SaaS. If that involves a different employer or renegotiating their compensation package to add in bonuses... well see it as practice.
If you are working full time already, how will you find time during the day to do these calls on the side unless you work remotely and are doing it on your current employer's time ? Sales calls do not happen after 5 PM or weekends so you will have to juggle. What if you have a meeting set at the same time ?
Also, it takes more than just a few calls to close deals. You have to follow up, you have to engage with stakeholders from the client's side, may be do another meeting etc. It depends on the product you are selling and the sales cycle. You cannot just wing the sales calls.