Hi Op, i suspected that MIL had worked , this is why i always cringe at the "what did MIL do " comments on these types of threads.
Ultimately, you have made a choice to work p/t which i fully respect. I would respectfully say though that this is the choice that you and your DH made and a choice which has subsequently had a financial impact on you both. Really, having made that choice, you need to standby it and wait until you are in a better monetary position to afford new cars etc. Obviously , if your DH were to work longer hours, take a 2nd job etc, even more of the childcare and hse work would fall to you as he would simply not be there to do it. I am not sure want you want here? Swings and roundabouts.
I do think your PIL have been generous and i suspect had MiL not worked as she did ,they would not be in the position they are now to help you both, as they have. Their contributions have improved both of your qualities of life and are now subsidising you working P/t however frustrating you may find acknowledging that. We do not fully understand the dynamics at play here, also what DH does or does not do around the house , so we have to reply based on what you have said here.
As pp said many single women work F/T , raise kids and do all hse work of which i am one. I am not quite sure how to read your comment about your MIL leaving your DH in childcare. I sensed it may have been meant with negative connotations but that could be just me , so genuine apologies if that is not case. I do appreciate that working full time , solely running a house and being a single a mum is not for everyone though. ( Just as a caviat my eldest son was in full time childcare since he was tiny. He is grown up now, has a fantastic job as a hardware engineer, recently bought his own brand new car , house shares with his workmates and has not ever asked me for a penny to date.).
As i said, i respect the choice you have made and understand your reasons. However, i do not feel that those choices should mean that your inlaws are paying for big ticket items and other bits that you clearly need in order to function and travel around.
If it were me i would not take any more money off in-laws from now on and would explain to DH that i expect his support in this and that you are a unit in this, together.
This will immediately solve the issue of the way you perceive the slights from your PIL. Yes, it will most certainly affect your standard of living until you have returned f/t but this is the choice you both made and if you are receiving hand outs, its not an independent choice as effectively you are allowing/ in need of others to fund your shortfall. Many women leave work to be sahm's or reduce hrs to part time but they live within their means and accept there is a financial consequence in doing so.
I think you and your DH both need to accept that you and your DH simply do not not earn enough singularly or jointly to allow you to work P/T and support the standard of living you would both like. You therefore have to cut your cloth to suit your measure. Your expenditure is greater than your income. You both need to start living within your means and wait and save for the things you want and need.