Firstly, you are a smart woman as you have outlined all of the issues that you will face being a stepmum. Most of us went into step parenting without your level of awareness. I genuinely thought that it would get easier but it hasn't. Time doesn't make the issues go away - if anything your tolerance and frustration grows.
DH was a disney parent (at the outset) and this continued for many years.Eventually he agreed to counselling to get over his inability to properly parent (guilt, fear of rejection from DSS). It has worked and DH is now a much better parent but getting through it was extremely tough and costly.
We also had the ex issue - I naively assumed that if we were good people (I wasn't cause of breakup, ex had affair) then in time the ex would settle down..Wrong..no matter what DH "gave" it was always an issue. The saying "you teach people how to treat you" is very true. We eventually had to go to court to establish boundaries and contact - which again has worked but has been painful, stressful and costly.
We have had a child and that experience is challenged by being in a blended family. Finances are tougher - the demands from DSS grow and we end up spending considerably more on DSS then we do on our other children.When money is tight that can be very hard to deal with as I feel our DCs should have equal treatment but there is no way that we can spend 15% on each child like we do with DSS.
Being a parent is rewarding however being a step parent is about giving for very little reward. You do have to accept that you give for little or no return. To survive as a couple you and your DP need superior communication skills, you have to agree on the major issues, such as parenting styles, finances and how to create boundaries with the ex. If you don't agree it will always be a source of major frustration and you will will end up feeling resentful. DH & I have to work at being a strong couple, it isn't easy and if I'm honest it's not a life that I would recommend to friends.
Your instincts are telling you to hold off committing to this man. I think you need to listen to those instincts. 3 years is time to have made progress, if he hasn't made you feel more secure, in that time, about the future then I think nothing will change.