To be clear, dont mind order of venice and vienna.  Will probably fly home or come back via cherbourg or st malo as live in poole so easy for ferry.

Paris is avoidable but it'd take all day… let me add an example:

- TER Cherbourg - Caen 06:18 - 07:29

- TER Caen - Tours 07:46 - 10:47

- TER Tours - St-Pierre-des-Corps 14:54 - 14:59 (I think there's a tram as well)

- TGV St-Pierre-des-Corps - Bordeaux St-Jean* 15:17 - 17:42 10-20€

- TER Bordeaux St-Jean - Agen 18:30 - 19:52

VS

- TER Cherbourg - Paris St-Lazare 06:46 - 09:57 1.70€

- métro

- TGV Paris-Montparnasse - Agen 11:11 - 14:22

*There is an earlier TGV at 11:05 from St-Pierre-des-Corps but it'd mean a 7 min connection in Tours. Is it possible? Should be. Perhaps I'd book both TGVs and refund whichever you don't end up taking -> possible until 30 min after departure.

I'll answer to your other questions too.

I see 2 main options (Paris - Germany could be one too but let's avoid it). Either via Switzerland or via the Mediterranean coast. Option 1:

- Intercités Agen - Marseille St-Charles 07:31 - 12:37 10-20€

- lunch break

- TER Marseille St-Charles - Nice-Ville 14:57 - 17:36

Night around Nice. Day 2:

- TER Nice-Ville - Vemtimiglia 07:48 - 08:41

- IC Ventimiglia - Milano Centrale 09:10 - 13:00

- FR Milano Centrale - Venezia S. Lucia 13:45 - 16:12 (depending on your travel date that journey could take longer due to works)

Option 2:

- TER Agen - Toulouse 05:49 - 07:10

- TGV Toulouse - Lyon Part-Dieu 07:39 - 11:48

- TER Lyon Part-Dieu - Geneva 12:38 - 14:46

- IC5 Geneva - Zürich HB 15:15 - 17:56

Night in Zurich (Lausanne or Bern could work too) and direct RJX to Innsbruck and Vienna the following morning.

Or night train to Vienna or Bregenz - Vienna cheap and shiny new night train.

Personally I'd choose the connection via Switzerland for the way back and then head to Strasbourg/Paris.

Btw I might suggest a 1st class pass, quite enjoyable on these long rides.