I advise my clients to treat the renting of property as a business. In your case, that means two things:
1. You must determine if you are willing to go through the hassle you are as compared to the hassle of finding a new tenant and who may not care for the rental in the same way. Generally, late payers never get better but worse.
2. If you do decide to start eviction, use an attorney. While you could train yourself to do it properly, it is technical and mistakes can be costly. A standard eviction can be done through an attorney quite reasonably. There is a lot of competition and an attorney will usually give a price for the service so you know what will happen.
If you decide to not evict them now, you need to train them to pay on time. Send a letter describing the problem and how it is difficult for you as well. Tell them you simply cannot wait past the due date without exploring your options again. Then, when they pay on time immediately after the letter (hopefully) give them a biscuit. A nice box of candy, bottle of wine, some free tokens for the washing machine--whatever small gift you think they might like that does not really affect your bottom line. Then, next month, do it again. (If on time.) Then, skip a month (even if on time). Then reward the month after that. You see, you are training them to pay on time. Sure, if they don't have the money, they won't change their habits for a biscuit. However, many just wait and hold the money in case something else comes up in the hope to shift the risk of life to you. After the habit is made, when paid on time, roll a die and if a 1 comes up, reward. (Or whatever number(s) you think will work.) It's just like training a dog to sit. You are trying to make good habits. Look to behavior modification positive reinforcement for why you progress as I just said.
Some may find that is silly. But, if these people are otherwise good tenants, 50-60 bucks a year in biscuits is way cheaper than finding a new tenant and the risk the new ones are the type to smear feces on the walls.