Is it feasible to travel from Ubon Ratchathani to Savannakhet for a spouse visa renewal on the same day?

You'll have plenty of time. Buses/ mini vans about every 15 mins from Ubon to mukdahan from the ubon bus terminal, it takes about 2 hours, around 140bt. The border at mukdahan/savanakhet closes at 10pm. A taxi from the airport to the ubon bus terminal should only cost about 100bt. Plenty of buses from mukdahan bus station to the border and onto savanakhet, about every 30 mins.

from mudahan immigration there is shuttle bus - 50baht will bring across the border / keep the ticket stub & after clearing immigration you can use it to take the next bus to the main bus station where you can find guest house & the consulate. attached is the daily timing from mudahan to savannakhet.

Going back to Ubon Airport I ended up taking a taxi -- a bit expensive but I felt better knowing I would make the flight time with ease ...

A quick taxi to the Ubon bus station from the airport will help. I did this not long ago and was able to catch a bus to Mukdahon within a half hour .. there are vans and taxis there too ... Taxi to Mukdahon - less than 2000 Baht - maybe 1500 as an alternative.

The Thai Consulate in Savannakhet has actually been opening at 9:00 a.m. (not 8:30 a.m. as posted on the sign). Lines have not been long.

While in Savannakhet, I had to go to the Lao Immigration station at the bridge to get documents couriered to me and come back ... I didn't get in to the line at the Consulate until 9:45 and only two people were ahead of me... They take applications until 11:00 ... So you have a fair amount of time to deal with travel..

I recommend in the late afternoon / early evening - when going to Savannakhet to stop in Mukdahan at the Kiengpiman Hotel - stay the night... Nice place - cheap at 400 Baht. The next morning the manager / staff will get you a taxi to the entrance to the Mukdahan > Savannakhet Bridge early in the morning - where you have to get on a bus - stop at Thai Immigration - process out -- then ride the same bus the rest of the way to Lao Immigration ... (allow 30-40 minutes processing and transit time. A taxi or tuk tuk from Lao Immigration to the Consulate takes about 10-12 minutes drive time ... Then after turning the Visa Application in - go to the Avalon Hotel by Tuk Tuk .. rooms up to about 680 Baht equivalent... Nice place ... not very far from the Thai Consulate for retrieval of the Passport with Visa the next afternoon after 2:00 p.m.

Of course you can go on to Savannakhet that evening and not stop in Mukdahan - but not much advantage in doing that.