I have EU citizenship through my dad so we are also considering retirement in Europe where COL and QOL are higher than US. I estimate our retirement needs will be around 60-80K, yr to live really well. How much do we need to get to to achieve that goal?
I would work on tracking expenses to settle on a more concrete number as you get closer to retirement but to give you a ballpark answer: for 60k a year at a 4% withdrawal rate you would need $1.5MM and for 80k a year you would need $2MM.
You have $100k each for the kid's college in a 529 plan. That should be more than enough to cover college expenses by the time your kids reach college age (assuming another 10 years at roughly 8% growth a year you should have over $215k by the time all is said and done for each child.
As to your personal retirement/Financial Independence goals:
1. General Advice is to avoid paying off the house faster as your return (assuming you invest the difference) will be significantly better than 3.5% interest you are paying on the loan. This should be weighed of course against how much you and your spouse hate having debt and the "need" to continue saving vs. just having more assets in the bank. Everyone's situation is different but from your first paragraph it sounds as though there is a very strong dislike towards debt from you.
2. You have 300k in ROTHs and 401k and 50k in taxable so 350k total. Taking your numbers and doing simple "back of the napkin math" If you take home 250k post tax (I'm assuming this doesn't count your 401k and ROTH savings either) minus your 80k spend gives you 170k to save each year so in 10 years that's $1.7MM to add to your 350k giving you $2.05MM (With no growth of course)
3. This is of course way oversimplified but I just wanted to illustrate that you and DH are in great shape and simply continuing to plug along you will reach your FI numbers well before you thought. (Approximately 8 years from now based on current numbers given) and if you continue to work then great that money can be saved as "cushion". As for how to invest it, I would definitely recommend just selecting Index funds based on your preferences and then building a massive stache with that.