Yes, Maemo Leste is a complete distribution, based on Debian/Devuan with mainline Linux. The general idea behind it is that by basing it on top of a modern, actively developed distribution and kernel, once ready, it will require minimal maintenance so it won't suffer from bit rot like Fremantle.

It might seem like overkill but part of the problem with Fremantle is that it is so old now that a lot of new software may not build against it because it may depend on libraries much newer than those bundled with Fremantle. I'm not saying that building AutoSSH for Fremantle can't be done, it's just something to consider. If you still want to go down the Fremantle route, the one thing I would recommend (if you're not doing it already), is to make sure you're using Scratchbox to build AutoSSH.

Leste is not actually that difficult to install. You don't need to replace Fremantle, it can be configured for dual boot. All it requires is to copy it to an SD card and to install and configure U-Boot. More info here:
https://leste.maemo.org/Nokia_N900#Installation

I would guess that your Python script won't work without modification. Leste uses the actively developed oFono telephony stack so your script would probably have to be adapted to use that. You should be able to use unicsy_demo to test SMS. It's written in Python so hopefully that will help you to get your script working.