The groups end up very polarised and I don't think it's helpful. For example, they will probably suggest that you replace them all with something like Joie Stages, which isn't necessarily any safer.
Remember that any car seat you buy today is going to be miles better than something that was on the shelves 30 or so years ago.
Yes, it's true that the generic/budget brands tend to have fewer safety features when compared to the top end brands who make safety their USP and are at the cutting edge of newer technologies, but all the "new" safety features trickle down and become part of the basic seats eventually. For example, even cheaper car seats these days tend to have deep side head protecting wings, whereas this did not used to be a feature on more basic seats. Things are improving all the time and I do not think it is helpful to shame people over using a brand which is not on the approved list of some group.
Your DC1 is quite a while off the limit - probably a year or so away, so no rush to change them. Are some of the seats you have all stages type seats?
The seats are best off used rear facing, especially for the 15 month old. For the 3yo it's less of an issue to be forward facing, but if worried about the quality of a cheaper seat and RF is an option (ie not over weight limit), then it is an easy way to instantly increase safety.
It's important to ensure that the seats are used correctly as per instructions. If they have top tether, make sure this is attached properly. Make sure the harness/headrest is set to the correct position - this is usually as close to the top of the shoulders as possible. If child is between two slots, go up one for FF and down one for RF. Don't put them in the car in bulky coats, do tighten the harness so it's snug to their body - show your PILs too. This is more important if FF.
If you have the option to increase space between the child seat and the seat in front, this is also useful to do, but again, a nice to have, not essential. In the vast majority of cars there is already enough space to cope with the expected movement in a crash as long as the seat + tether is correctly fitted and the harness correctly tightened.
In general I don't think that you would gain a massive amount in safety from replacing the seats, certainly not compared to the cost. Keep at least the 1yo rear facing unless there's extreme car sickness or something, make sure you double and triple check the instructions, use the seats as best you can. No need to put yourself into financial difficulty. (IMO).