SPY and SPYM

I have been investing in SPY until now and that has been giving me good returns so far. I recently started hearing about SPYM which has something like low expense ratio but lower “liquidity” also compared to SPY. I know that SPYM is not suitable for day trading. But is it suitable if I want to invest every month or week ?

It has something like a low spread compared to SPY. So does that mean that I will have to pay a high transaction cost if I invest every week or month or does that mean that in the case of a sale event, the sale will be executed after few hours ?