can we use our phones normally just as at home?
Not without a charge. From the FAQ
"You get all of the features of T-Mobile ONE Plus with two added benefits. Get stateside international talk and unlimited 4G LTE mobile hotspot for just $25 a month when added to a T-Mobile ONE line."
"Stateside International Talk lets you keep in touch with friends and family outside of the USA. Get all these features when you call or text from the U.S., Mexico, and Canada:"
Stateside International is for calls made from the US, Canada or Mexico. When in France/Austria you will be charged international rates for those areas. Go to this link to check those rates. Simple Global | Travel Abroad without Roaming Fees | T-Mobile
but will both our phones be browsing on LTE at 4G speeds with no additional charges?
No, by default (assuming that you have T-Mobile One) you will have data speed of 128kps. With the T-Mobile One Plus or T-Mobile One International this is doubled to 256kps. Your phone may show LTE but you are throttled back to 128 or 256 depending. If you log into your My T-Mobile account then select plans and listed for each phone you should see "Stay connected to the web with additional high-speed data. Add a data pass". This will allow you to add high speed data passes for international data. I find the T-Mobile One International poorly stated as it certainly sounds like it allows calls when abroad. I also found this FAQ link that will help. With T-Mobile ONE will I get the same high-speed network experience internationally, as I do here at home in the U.S.? Ignore the $5 cost listed there as it was recently increased to $10.
We recently travelled using the 256kps and found that it was sufficient for web browsing and email and with the T-Mobile One Plus you will have 256kps. Any phone calls in or out will be charged the rate for that country. We were in Peru and the rate was .20 per minute for any calls not using WIFI calling from the hotel. Be careful with voicemail. We had two charges for an inbound calls from the T-Mobile voicemail number. I do not recall receiving a call nor did I check voicemail but when I returned I had the charge. We were able to use Facetime, Whatsap, and Google Voice to make calls while on roaming data and quality was ok. We found that Whatsap worked best for sending messages with the slow speeds and calls. If not a user of Whatsap you will need to load the app and then make sure that anyone you plan to call or call you also has the app loaded as you can not call regular phone numbers with the app.
Another option which needs to be confirmed as we did not do it would be to use T-Mobile Digits. If you add the T-Mobile One Plus to your line for $10 per month you will get a free Digits number. You will need to confirm but I believe this Digits number will use a data when calling out. If this is true it might give you a number for people to reach you and the ability to call back to the states to a DID number.
I hope that this helps and is accurate for your needs. We used our phones in Peru for 2 weeks like we would in the US outside of making calls and it was a great feature to have.
I'm confused by your question. Both the older "Simple Choice" and the newer "TMobile One" plans have had intl roaming in 130-140 countries for a few years. We have used it in 13 or 14 countries. (Our data speed is usually 110-112Kbps.) Sounds slow - but email, Google Maps and even some radio streaming work fine. You can not set up a hotspot, AFAIK. Of course you can't, at those speeds, stream video.
I don't see how paying extra for faster speeds would help most people. My 2 cents (plus tax).