I desperately need some advice before recommending a wrong choice to our suits, choice is between exchange onprem and SE.

So im at my first job as a sysadmin and im loving it so far. Its a small nonprofit company with really great people and its the first time since i can remember that i actually enjoy what im doing. So if this is the wrong place to ask im sorry but im desperate and dont want to fuck things up.

Long story short, our Exchange 2016 went out of support and we need to make a descision on how to proceed. (We already upgraded to SE so no current vulnerability).

We are a tiny inhouse IT and have a IT Service Provider for bigger projects. Their offer was to basically buy 250 Licenses for M365 Business premium and to migrate our current on-prem exchange to the cloud, as well as configuring entra ID and stuff to work with our onprem AD.They quoted us about 40k in total with about 15k yearly for license. We are a non-profit and get a ton of discounts on these type of things.

Now the only alternative we have have for different reasons is to keep running our Exchange as an SE on prem, and buy 250 licenses for Office Buisiness Standard, which should should cover the necessary Exchange CALs.

I calculated the costs after 10 years, and basically migrating to the cloud would cost us almost 200k while the onprem option would cost us roughly 100k.

We are a non-profit, money is ALWAYS tight. I KNOW that if i show them these numbers, they will obviously choose the on-prem version. Now im just scared that im either overlooking costs or missing something that would make it cost way more in the long run. Like we will propably still need to hire a supporter to help us with the licenses and stuff, but i feel like all that is gonna be so much cheaper than paying for a whole cloud migration.

Am i wrong? Am i gonna regrett it or run into trouble some time later if we keep our onprem exchange?